Brass Spencer 20x N.A. .050 Objective Spotted

[desertrat]

This might be a good addition to anyone using an A/O or Spencer Lens Co. 160mm scope. A couple of years ago I bought an older version of this objective with a very low serial number in the 11,XXX range. I'm guessing it was made around 1905 give or take a few years. No one wanted to take a chance on it so I grabbed it for a low price.

I was amazed at its performance for an uncoated antique. There is a little less contrast than my coated 4 Series A/O 20x apo .060 N.A., but stopping down the condenser just a bit further brings contrast up to a satisfactory level for me. There is some field curvature, but not bad, and adjusting the focus makes the edges of the field almost as sharp as the center. And this is with Cat. 146 wide field eyepieces that force the objective to cover a wider field than it was intended to.

This objective occupies a place on my 4 Series Microstar turret between the 10x and 43x objectives. It gets a lot of use when viewing pond water and stained permanent whole mounts when I want a closer look from the 10x. I don't use the 43x much anymore for the afore mentioned purpose because the antique 20x has a wider field, better depth of field, and almost as good resolution as the 43x.

I haven't done business with this seller, but he has a 100% feedback rating and deals in microscopes and accessories based on his feedbacks.

From his description:

Here we have an Excellent Spencer ( AO ) Buffalo 20 X Semi Apochromat Objective ( 8mm ) with a Numerical Aperture of 0.50. This Objective gives a beautiful Image and Is very nice to add to an Antique Spencer or A.O. Microscope.

I don't know if I could call mine a semi-apo, but it performs very favorably compared to the coated 1950s vintage achromats on the 4 Series. And the N.A. is a bit higher than the 0.40 of the typical 20x achromat. Mine also outperforms my LOMO 20x N.A. 040, and my set of 33mm parfocal LOMO objectives otherwise perform pretty well.

https://www.ebay.com/itm/Spencer-antiqu ... SwblFakyxj

P.S. I forgot to add the brass antique is parfocal with other 4 Series objectives on the nosepiece.

[Charles]

I think the N.Ap means Numerical Aperture instead of Apo or Semi-apo. I believe all AO 20X objectives have and NA of 0.50.

[apochronaut]

He has lowered his price for the current listing. I did have some communication with the seller about a month ago and he isn't all that knowledgeable. His assumption that it is a semi-apochromat is purely HIS assumption, based on the fact that it must be, since it has an N.A. of .50 and any 20x objective with an N.A. of .50 has to be a semi-apochromat.....apparently.
Anyway, Spencer did not mark their objectives as achromat. They just marked the fluorites, as fluorite and the apochromats as apochromat. I don't think the term achromat arrived on the barrels until the term planachro appeared in the late 60's. Charles is correct. For a number of years that N.AP. designation appeared on the barrels.

I agree, desertrat, ( jeez; i almost spelled that dessertrat.....boneless, please), those older Spencer objectives are usually really fine. I have a version of that 8mm, 20X .50 objective with a .20-.50 iris diaphragm in it, which I assume was so built for micrography, and it doesn't want too much for contrast, resolution and planarity.

I have never seen an 8mm Spencer fluorite, nor one catalogued. That doesn't mean that they did not produce some, since they actively pursued custom orders but for decades, their 8mm achromats were sufficiently good that a fluorite version might just have been redundant.