ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
261 
or senms. When used in tlie 1/4 1 dreaded making them more than 
all others in the objectives, often having to throw them away, owing 
to their having such defects. Shortly after our removal from Cauastota 
to Geneva 1 made a 1/12 water im., containing one spar lens; but that 
is the only one since I was a boy. When Dr. H. J. Detmers, of 
Columbus, O., visited me at Cleveland (about the time of the meeting 
of the A.S.M. at Detroit) I showed to him the old book containing 
the records of all these objectives as kept by my father, and among 
them the 1/8 marked E. K. B., also 1/8 containing a spar lens, which 
was made as far back as 1851, about the time my father began making 
objectives of large angle, showing how early in the fight he appreciated 
the valuable optical qualities of fluor spar in the construction of 
objectives. The angular aperture of this 1 /8 was not given, but I can 
readily see that it could not have been less than 160°. 
These are, I think, all the facts necessary, but I may have made 
too much of it. It has been hurriedly written oil' and is not in proper 
shape for publication, but you can cull out the facts and arrange them 
properly for publication in the forthcoming proceedings. I shall esteem 
it a great favour if you will do so, as I look upon the facts as of value 
in connection with the history of the Microscope. 
I trust you will pardon the use of the pencil, for I have been 
anxious to send this by the first mail. Please let me know if it reaches 
you in time. — Very truly yours, H. E. Spencer. 
Fluids for Immersion Lenses.* — Dr. A. C. Stokes remarks: — 
“Usually the only immersion fluids at the microscopist’s command are 
water, cedar oil, and glycerin made dense by dissolving in it either 
cadmium sulphate, zinc carbolate (sic) or some other salt. With homo- 
geneous-immersion objectives, or those using an immersion medium with 
a refractive index as nearly as possible that of crown glass, so that the 
cover, the immersion medium, and the front lens may form one homoge- 
neous combination with these objectives, water of course cannot be used ; 
so that the microscopist must have resinated cedar oil or the glycerin 
solutions just referred to. But to obtain the best results from these 
first class homogeneous-immersion objectives it is important that the 
immersion fluid should have the proper refractive index, that of crown 
glass being 1 *5, of cedar oil 1*515, the glycerin fluids varying in a way 
that the microscopist has usually no means of finding out. Prof. H. L. 
Smith has devised a simple and successful little instrument for the 
measuring of the refractive index of such liquids, but, so far as I know, 
it is not in the market. The microscopist must therefore rely on the 
optician, that sometimes by accident plays him false, and so deprives 
him of the best that his objectives can do. I have recently had an 
experience with these substances that has taught me, if not wisdom at 
least caution in blaming my objectives or even my own lack of manipu- 
lative skill. 
A certain homogeneous-immersion objective, of not large numerical 
aperture, was said to be able to resolve Amphipleura pellucida well and 
easily. I made the attempt, and failed, after several hours’ work with 
the lens, using all the care and skill that I possessed. The immersion 
Microscope, xi. (1801) pp. 311-4. 
