860 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
my standard of excellence, or liking. What I wanted to attain to was 
simply this : to be able to look through the tube as though there were 
no glass or medium in it ; for it to be absolutely transparent or trans- 
lncid, absolute distinctness of detail, edges of objects absolutely sharp 
and defined, the same with bubbles of air in mounts, to be able to see 
the surface of the finest polished glass-films with any power, and to see 
the depth of an object at the same focus. After many experiments with 
diaphragms, at last I said to myself: achromatic lenses are made of 
flint and crown-glass; the refractive index of flint is 0*026; its dis- 
persive power, 0*052; some specimens refractive power of 0*029, 
dispersive power, 0*048; that of crown is 0*038 dispersive power, 
0 * 018 refractive power. The refractive index of Canadian balsam is 
0*021, dispersive power, 0*045. The refractive power of castor-oil, 
0*018; dispersive power, 0*036. After reckoning up the probable 
results of gain from these indexes, I concluded that after opticians have 
corrected the colour and achromatized, or made achromatic their lenses, 
they required to be further balanced and more nicely balanced than they 
have been in regard to their refractive and dispersive power. On 
experimenting, I found that in the Microscope a material element was 
the place of its being used, and that although Canadian balsam was 
more whitish to the eye, it dispersed nine times and refracted three 
times too much to make an even balance. It does not give good results 
when used on object-glass in any way. On carefully cleaning with 
spirits of wine an achromatic lens of the same size as the body of the 
Microscope-tube, and cementing them together with castor-oil and fixing 
them at the end of draw-tube, judge of my delighted surprise when I 
found I had thereby attained more than everything I wished for with my 
Microscope, and equally attained it whether I used 2-in., 1-in., 3/4-in., 
1/2-in., 1/4-in., 1/8-in., or 1/25-in. object-glass. The tube of the 
Microscope was absolutely translucent, and I could look through it as 
if it contained no glasses whatever. Field of view absolutely flat ; 
focus of lens at end of draw-tube 8J in. ; full length of Microscope-tube 
a little over 1 foot. Castor-oil as an immersion does not act properly. 
On testing Canadian balsam with a telescope of 2 feet and lj-in. object- 
glass, and comparing it with the object-glass cemented with castor-oil, 
the latter gave incomparably best results. I can see the flies playing 
in the air between the line of sight and the buildings or conservatories 
in the distance.” 
On this we need make but few remarks, for, as is well known, 
the action of a lens at the end of the draw-tube has been advocated 
and trie l ad nauseam. Dr. Piggott’s “ Aplanatic Searcher ” is practi- 
cally one of the instances ; it was about to revolutionize the practical 
optics of the world, only unfortunately it failed and is nowhere to be 
found to-day. 
It is possible that the “ 8^-in. lens” at the bottom of the draw-tube 
may, by reducing the power of the optical combinations used, slightly 
increase the light and so produce what by contrast is a sharper picture, 
but a mere change to a lower eye-piece would have done the same. 
The refractive indices appear to be wrong ; but, neglecting this, the 
film cementing two lenses together is of necessity very thin and has 
parallel curves ; therefore assuming that the writer is correct in his 
