ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
393 
practical optics. Tlie great transparency of these minerals for violet 
aud ultra-violet light has recommended them for spectroscopic purposes, 
but their chief use depends on the specific property of non-tesseral 
(crystals of double refraction, by which results are efi’ected which are not 
to be attained with an amorphous substance, such as glass. Tesseral 
crystalline minerals which, so far as their optical properties are con- 
cerned, are like glass, have already been made use of for optical 
purposes. Brewster and Pritchard, within the last forty years, recom- 
mended and employed the diamond and other precious stones of un- 
usually high refractive power in Microscope lenses. Attempts in this 
direction, however, have brought no lasting gain to optics, and must at 
present be considered as wholly given up, the simple Microscope formed 
of uncorrected lenses being relegated to a subordinate use. Quite 
different points of view are now kept under consideration in judging of 
optical resources for the continued improvement of the compound 
Microscope. For, in face of the refinements which, in recent times, 
practical optics has kept in view, the estimation of the materials used in 
lens-combinations has altered in direction ; it no longer looks at the 
greater or less perfection of its fundamental effect, which depends of 
course on the refractive power, but it has turned to the consideration of 
the degree in which the properties of these materials facilitate and 
advance the neutralization of the unavoidable subsidiary effects — 
spherical and chromatic aberration. 
From this point of view a material which, from the standpoint of the 
efforts of Brewster and Pritchard, would appear very unprofitable, viz. 
fluor-spar, becomes of special interest at the present time for practical 
optics. This is because it offers unusual advantages in respect to the 
neutralization of those subsidiary effects. Fluorite possesses an abnor- 
mally low refractive power; the index for sodium light is only 1*4338, 
and is thus considerably lower than that of crown glass ; its use as a 
constituent of a lens-system is therefore, in respect to the fundamental 
effect, relatively disadvantageous. However, with many lens-combina- 
tions, such as those used for the Microscope, there must be a difference 
of refractive indices between media in contact and with equal curvature 
of the bounding surfaces in contact to remove the spherical aberration ; 
it is on the amount of this difference that the compensating effect in 
resj)cct to the spherical aberration depends. The lower the index for 
the first medium, the greater the amount of this difference, and the more 
perfect the compensating effect which is to be attained by the addition 
of a second medium of given refractive power. So, also, the lower the 
index for the first medium, the lower that of the second, when a certain 
given difference is to be maintained. If, for example, in a cemented 
double lens — as used in the Microscope — an ordinary crown glass of 
index Wp = 1*52 serves as the one member, and the removal of spherical 
aberration requires a difference of refractive powers of 0*20 on both 
sides of the cemented faces, then the above consideration shows that 
there must be connected with that crown glass a second lens with index 
1*72, and consequently one made of a very heavy, strongly dispersive, 
flint glass. Suj^posing, on the other hand, the first member to be a lens 
of fluorite, then the required excess of refractive power of the second 
member would be given by an ordinary flint glass of 1 * 63, which for 
