A thought about size

[DonSchaeffer]

By general agreement, psychophysical measurement has established that the smallest objects visible to the naked eye are about 100 microns. I think it would be interesting to use these units as a standard size measure, calling them limens. For instance, most of the ciliates I've seen would be around .2 limens.
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https://www.quora.com/Whats-the-smalles ... e%20Amoeba.
https://en.wikipedia.org/wiki/Psychophysics

[janvangastel]

I once wrote an article about that for a Dutch magazine about astronomy. Very interesting topic.

Jan.

[PeteM]

I'd guess visual acuity varies vastly between people, even if we lopped off the extreme couple percent at either end of the curve. Looking at a standard optometrist's eye chart (those rows of letters to guess at), many of us have a difference of as many as three or four rows between our very own two eyes.

[Tom Jones]

Per your Wiki link:
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation"[1] or, more completely, as "the analysis of perceptual processes by studying the effect on a subject's experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions".[2]

Psychophysics also refers to a general class of methods that can be applied to study a perceptual system. Modern applications rely heavily on threshold measurement,[3] ideal observer analysis, and signal detection theory.[4]

You're suggesting measuring actual size with "units" based on perception of size. That makes no sense. Put another way, if I'm looking at Jupiter in the night sky, and it appears to my unaided eye to be about twice the minimum size I can normally perceive as other than a point source, does that make Jupiter 2 limens as well?

[hans]

You're suggesting measuring actual size with "units" based on perception of size. That makes no sense. Put another way, if I'm looking at Jupiter in the night sky, and it appears to my unaided eye to be about twice the minimum size I can normally perceive as other than a point source, does that make Jupiter 2 limens as well?

Since Don is talking about "smallest objects" the 100 um number is presumably based also on typical least distance of distinct vision, not just typical angular resolution, so I don't think there is anything fundamentally nonsensical about the proposal. The diameter of Jupiter is 1.4 teralimens. Could this proposal be sign that Canadians are finally recognizing the folly of SI and yearning for the advantages of a more imperial/US-like system?

[DonSchaeffer]

I'm just talking about a convention--nothing substantive. I think describing size relative to the generally accepted visual limen would be a good idea.

[MichaelG.]

It’s an interesting idea, Don ... but [sorry] I think it’s fundamentally flawed:

Limen is a perfectly good word, with a specific technical meaning which differs from your proposed usage.

It comes from psychophysical experimentation and is broadly ‘threshold value’
The important things being that it
(a) can apply to any stimulus, and,
(b) varies with the prevailing conditions

It acknowledges that, in perception, our responses are not actually ‘binary’ but ‘three state’
i.e, Not Yes/No but Yes/maybe/No
... The limen value is that at which we are 50% undecided, or at which [if forced into a binary choice] we are equally probable to choose Yes or No.

It is perhaps most widely known by ‘Subliminal’ ... which means ‘at a value below the limen’
[noting here that how far below is irrelevant]

In the usage which you propose, the best vernacular expression might be ‘barely visible’
... as in “the subject of this photograph is barely visible to the naked eye”

It’s a nice concept, but limen is essentially a qualitative measure not a quantitative one.

MichaelG.
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Edit: Tom Jones has already covered much of this, but I needed to ‘get it off my chest’

[patta]

I'd suggest to express the limen as inverse fraction, or the magnification needed to see the object/feature.
In the example with the limen at 100 micron and the ciliate 20 micron, it will be a 5 limen - I need 5x magnification to see it.

A similar concept is the apparent magnitude of the stars, which was a naked eye approximate/perceptive but quantitative measurement.
Magnitude 6 = barely visible; magnitude 5, discernibly brighter than 6; magnitude 7, not visible to naked eye.

[DonSchaeffer]

I like the analogy with star magnitude. Anyway, no one is going to adopt this. It's just an idea.

[MichaelG.]

For what it’s worth ;

Stellar Magnitudes are a ‘measure’ of brightness, not size ... and they were originally grouped by whole numbers [rather like a bar chart], with ‘first magnitude’ being the brightest grouping.

Note: This has since developed into a scale, but that was not the original intent.

MichaelG.

[75RR]

By general agreement, psychophysical measurement has established that the smallest objects visible to the naked eye are about 100 microns. I think it would be interesting to use these units as a standard size measure, calling them limens. For instance, most of the ciliates I've seen would be around .2 limens.

If the original reason for DonSchaeffer's post is the difficulty in explaining just how small the small things we look at are when sending photos of animalcules to friends and family,

particularly when using microns draws a puzzled/blank look, I have had the same problem.

My solution was to use a dimension that everyone can visualize i.e. one millimeter (1mm), and fractions of it.

So 100µm is 1/10 of a mm, 200µm is 1/5 of a mm, 500µm is 1/2 a mm

For the sake of simplicity 50µm at 1/20 is about as small as you want to go when impressing the smallness of things, anything smaller is simply said to be less than 1/20 of a mm

[PeteM]

I like 75rr's notion of just getting used to mm and microns.

Another handy reference is the thin edge of a cover slip - around 170 microns.

The diameter of a human hair ranges from 17 microns to 181 microns per "The Physics Factbook." The typical diameter is around 50 to 75 microns. Chop that strand of hair to 50 or 75 microns length and many (most?) of us wouldn't see it.

A typical shop micrometer (measuring instrument) measures to .01mm (10 microns). A more accurate one can read to .001mm, but is typically only accurate to about .002mm (2 microns). This is about the limit of ordinary machining accuracy -- and also likely cover slip thickness measurement.

[MichaelG.]

Here’s a useful ‘benchmark’ image, courtesy of Wikipedia
... a 50 micron human hair, and a 6 micron carbon filament in the same frame:

https://en.wikipedia.org/wiki/Micrometr ... haarrp.jpg

MichaelG.

[DonSchaeffer]

That is interesting!

[janvangastel]

I like 75rr's notion of just getting used to mm and microns.

Yes, A kind of 'notion'or 'feeling' will come with time, or at least we think it will, because we get used to the terminology and develop some king of 'feeling' about it. In astronomy its the contrary: trying to comprehend how large things are and how far away. In the beginning one will try to calculate how many kilometers/miles one lightyear is (which is interesting), but after some time (we think) we have developed a kind of feeling about the distance of a lightyear and do not calculate the number of kilometers or miles any more. Even simpeler: when the euro was implemented in Europe, people calculated how much they had to pay for something in the old currency. But after some time the need to do that was not felt any more.

[charlie g]

Thanks,DonS. , for a thoughtful thread, for me terrific benefit gained from the folks who posted into this conversation. Thanks thoughtful forum members.

I agree with the sentiment that 'limen units' have too many variables packed into the eyes of beholders attempting to share with others 'limen units' for description of an objects size along any given axis.

I always rejected 'standard diameter of a human hair' as worthless.other than to give a range of landscape scale present in the microscopic world we manifest, My nasal hairs are much larger in diameter than my forearm hairs, etc. varieties of human hair vary in diameter greatly.

Another size of the landscape scale just beneath microscopic, for me would be the 'colloidal size particles' scale. Naked eyes frequently observe this colloidal size scale when a projection beam of light travels through a dark room in which someone has been smoking, or the situation when a sunlight beam enters through curtains with a small gap or hole, and into a room where the toast has been recently burnt too much.

We all have an easy learning curve to shop for jumbo chicken eggs vrs large eggs, vrs medium eggs, or jumbo shrimp vrs medium shrip. So too I think carrying the tool of the macroscopic worlds millimeter unit permits an easy learning curve for the microscopic worlds of micron size.

I at times wonder how Antonie van Leeuwenhoek , at late 1600's dinner parties at which he was the invited guest to entertain and to enthrall the high society guests, how did Antonie explained size scale to them as he passed around his microscope for them to get first views of the microscopic world! happy fall'20 charlie guevara, fingerlakes/US

[DonSchaeffer]

I chose the word "limen" arbitrarily. I think 100 microns is a convenient reference number for what would be visible to the naked eye (even if it is not always precisely correct--it is a consensus number like IQ 100 or 20/20 vision). When I post I might say this object is 20 percent of visibility. It needs to be 5 times bigger to be visible.

I'm proposing a convention that would be understandable and easy to determine.

Hee hee--I don't expect it to be accepted and I'm not aiming for a Nobel Prize.

[charlie g]

Again, DonS, congrats on a thoughtful thread..you 'keep it real'...and your shared thoughts , your shared microscopy appreciated.

All I say is..I sense no need to clutter shared microscopy with yet another layer of size units. I sense it easy enough to use the macroscopic unit "millimeter" to invite non microscopists into the optical level of all our microscopic world views.

All our worlds detailed pursuits require standard units to permit global agreement on what we are talking about, what size specs we are mutually working with...I see no need for a novel unit of measure at this time..there is no need for 'limens'.

BTW, I myself still can not get my mind to grasp really deep time....the geologic time frames of our dear globes approximate 4.54 billion years from initial accretion, then onto approximate age of our Milky Way home galaxy: approximately 10-12 billion years old. Currently the age of our (? our home universe?) universe is speculated as: 13.8 billion years old. I am comfortable with our earth-year time unit..but for 'light year distance/ size units', I appreciate our dear sols local stars vrs further distanced entities...I accept what I do not grasp..yet I enjoy visual sky gazing.

I really sense I am comfortable with microscopy world views! charlie guevara, fingerlakes/US

[DonSchaeffer]

We each have our own sense of wonder that keeps us searching. Good for you.

[Greg Howald]

Well Don, it's an interesting thought and you certainly got quite a bit of response to your thought. Its readily apparent that different opinions regarding your idea certainly abound. Adding a scientific term that is universally acceptable in this day and age is more difficult than passing a bill through the United States Congress, and we all know that is impossible right now, but it's not a crazy idea.
Greg