260 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
by looking through it from the back with a magnifier a hazy-granular 
appearance was noticeable, not due to dust on the back lens. 
Last March the objective was sent to the makers for examination 
and repair. It reached me again in July, as good as new, with the 
statement that the front lens had been slightly decentereil, and that 
the repair had been easily made, and was without charge. I have no 
other information upon this point, neither do I know what interpretation 
to place upon the granular appearance noted. There is certainly nothing 
of the kind visible now.” 
Fluor-spar Objectives.* — The following letter, communicated to the 
American Society of Microscopists, will be read with interest. 
Prof. T. J. Burrill, Buffalo, N.Y., Jan. 27th, 1891. 
My dear Sir. — Your favour j of 21st inst. came to hand this 
morning. Since coming to Buffalo my time has been so fully occupied 
that I had for a long time forgotten entirely the subject mentioned in 
your letter. As the time is so limited, and having no knowledge of 
how much space you had intended to give to the subject in the forth- 
coming publication of the ‘ Proceedings of the A.S.M.,’ I have thought 
best to state to you, as briefly as possible, the facts, leaving it to you 
to arrange them in proper form for publication, with whatever com- 
ments you see fit to make. 
During the summer of 1860, Dr. Bufus King Brown, who was at that 
time a resident of Brooklyn, New York, visited my father at Canastota, 
and during his stay there my father made for him a 1/8 objective, 
which was considered by all who saw it to be the best ever made up 
to that date. Although but a young boy at the time, I was greatly 
interested in my father’s work, and knew pretty well what was going 
on — hearing a great deal of talk — and remember well Dr. Brown’s high 
praise of the performance of tbe objective, but of course knew nothing 
of its construction until some years later. Some years later, after 
Mr. Tolies had removed his establishment to Boston, Dr. Brown became 
a resident of that city, and showed the objective to Tolies, who praised 
its performance very highly. 
The angular aperture of the objective was 175°. One of the systems 
contained fluor-spar, and it is on record in the formula that it was 
remarkably perfect in its corrections for both figure and colour, with 
both oblique and central illumination. In the years 1861 and 1865, 
I made lenses for quite a number of objectives, mostly 1/4 in., con- 
taining at least one lens of fluor-spar, and having apertures from 170° 
to 176°; but of course with very short working focus. In all these 
objectives, as well as in the one made for Dr. Brown, the spar lenses 
were cemented between others, as owing to its softness and liability to 
become scratched it was not considered safe to leave it in an exposed 
position. About the time that immersion lenses came into general 
favour in this country the use of the spar was abandoned, owing to the 
difficulty experienced in procuring that which was free from fractures 
* Proc. Amer. Soc. Micr., xii. (1891) pp. 248-9. 
+ This letter was received by me January 31st, 1891, after the preceding pages 
were in the press. Prof. Burrill informed me by accompanying letter that he had sent 
a request to Mr. Spencer for information as to the facts about fluor-spar objectives. — 
[Ed. Proc. A.S.M. ] 
