High-Powered Objective always hits the coverglass

[DonSchaeffer]

I can hardly ever use my high-power objective because it focusses below the level of the cover glass. It either knocks it when I move the slide or it doesn't focus. Are my cover glasses too thick or what?

[MicroBob]

Hi Don,
stronger objectives have small free working distances. Your cover slips should not exceed 0,17mm by much. I once bought chinese ones that were up to 0,25mm!
But most probably your preparations are not as thin as they should be. When you mount a wedged section part of the object will be too far away from the top surface of the cover slip. When you look at pond life you have to make sure that no sand grains or thicker plants or animals keep the cover slip from settling down on the smaller objects.
This would be a simple test object: A slide, some sharpie lines, a drop of water and one of your typical cover slips. Still problems with focussing?

Bob

[DonSchaeffer]

Thanks Bob. I did buy a box of concave slides. If these work it can solve the problem too.

[MicroBob]

The concave slides are not useful for high power work: The lower objects are to far away from the cover slip top for the correction of the objective. The concave surface also forms a kind of lens that tends to upset the image quality. But for low power observation of bigger water life they are very nice!

[BramHuntingNematodes]

A high power objective will be only able to get a focussed view of small things on the immediate underside of the cover slip. If the objective has a numerical aperture higher than or equal to 1 it will require a drop of immersion oil on top of the slip as well. It can seem strange having oil slopped on the cover slip and on your optics, and some people don't use high power objectives because of the additional hassle of cleaning. Also, make sure to never use oil on objectives not designed for it. Oil can get in and fog the lens if it jasnt been made with oil in mind!

[DonSchaeffer]

Thanks Bob and Bram