ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
255 
rate for the ordinary purposes of the microscopist, consists in making a 
fine mark with a diamond on an ordinary glass slide and cementing on 
each side of it the two halves of a large cover-glass, leaving a space of 
about 1/8 in. between their edges. The rectangular cell thus formed 
serves to hold the liquid under examination, and is covered with a very 
thin cover-glass. The tine mark is viewed with the highest power 
available, and the difference in focal adjustment for any two liquids 
examined is a measure of the difference of their refractive indices. 
Another simple device for testing the refractive index, due to Prof. 
H. L. Smith, is also described. The necessary apparatus consists of an 
adapter about 3/4 in. long, into each side of which a horizontal slot is 
cut. Through these slots slide two slips of crown glass (2 in. by 1/2 in.) 
having approximately the refractive index of ordinary cover-glass. A 
hollow is ground in one of these slips, and serves to hold the liquid 
to be examined. The instrument is graduated by using different liquids 
of known refractive index and focusing upon an object through a 1 in. 
objective, a mark being made on the rack-bar in each case when the 
focus is perfect. This apparatus was originally devised to test homo- 
geneous immersion media and has been called Prof. Smith’s Homo- 
tester. 
Optical Glass.* — Mr. J. R. Gotz discusses the properties and advan- 
tages to be gained by the use of the new glasses for optical purposes. 
Up to 1885 or 1886, in spite of the experiments of Harcourt and others, 
the manufacture of optical glass left much to be desired. Up to that 
time no means had been discovered by which certain errors of achro- 
matism could be eliminated. It was in 1881 that Abbe and Schott first 
commenced their experiments with a view to the production of new 
kinds of glass which would allow of the removal of the so-called 
secondary spectrum. 
The success which attended their efforts was attained by the produc- 
tion of improved crown and flint glass, mostly with mixtures of boracic 
and phosphoric acids, together with the addition of baryta, magnesia, and 
zinc oxide to obtain greater variations in refractive and dispersive power. 
Up to the present about eighty different kinds of glass have been put 
upon the market. These include several series of quite new glasses, 
such as the phosphate crowns, barium phosphate crowns, boro-silicate 
crowns, barium silicate crown, borate flint, boro-silicate flint, a special 
silicate flint, and a light baryta flint. 
The catalogue of these glasses indicates for each the refractive index 
for D, the mean dispersion from C to F, and the proportional or relative 
dispersion. The variety of these glasses is so great that for almost any 
special purpose a suitable glass can be found. Some of them are 
identical with glasses formerly made by Chance Brothers, of Binning^ 
ham. For photographic purposes the silicate crowns or flints, and also 
some of the baryta flints are especially serviceable, but the borate flints 
are unsuitable owing to the fact that they are injuriously affected by the 
atmosphere. In this connection the baryta light flints have proved of 
great value, for on account of the high refraction, lenses of this material 
can be ground with much flatter curves. 
* Anthony’s Photographic Bulletin, xxiii. (1892), pp. 624-8. 
