ria of Immersion Lenses. 23 
by the use of this immersion lens? Mr. Ross confidently adver- 
tises his immersion lenses as giving most brilliant definition, and 
with perfect good faith. And their superior action in many cases 
testifies most strongly to the inferiority of the dry objectives. The 
superb definition of Messrs. Powell and Lealand’s newly-contrived 
immersion lenses leaves little to be desired. But those eminent 
makers acknowledge that their best dry objectives are inferior to 
their immersion glasses. There must therefore be a cause of this 
inferiority. A study of deviation, especially when a minimum, may 
possibly account for some of the causes of this fact, now well ac- 
credited with advanced microscopists, and proved by the enwmera- 
tion (not merely by,the resolution) of Nobert’s XIXth band by 
Powell’s 1; ¢mmersion alone. The dry lens would not do it in 
Dr. Woodward’s hands. 
The method of obtaining the ratio of the lines of refraction and 
incidence between two media whose refractions are known out of 
air, can be obtained by dividing the one index by the other. 
Thus suppose it be required to find the refractive index between 
plate glass and water, ¢.e. between “, and“, . 
Refractive index required = “, +4, 
= 1500 + 1°336 
1 
= eal = 0800666 
Suppose, again, a ray of light emanating from a brilliant bead 
immersed in Canada balsam penetrates it at an obliquity of 43°; 
if now there is air on the other side of the cover, the ray will be 
entirely turned back by internal total reflexion, and it cannot reach 
the objective or the observer's eye at all. But now insert a drop 
of water, and it will then only deviate just about seven degrees, and 
attain an obliquity of 50° in the film of water. Again, every ray 
of greater obliquity, up to 62° 57’, will be able to reach the facet 
lens of the objective vid water, whilst only those less oblique than 
an angle of 41° 48’ can possibly reach the objective vid air. 
One advantage, then, of water versus air is that it enables the 
objective to gather oblique rays, striking it vd water which could 
not reach it vid air. 
Another advantage will be seen directly, that up to 100° ob- 
jective aperture the deviation caused by the water is three times 
less than that caused by air; and therefore eccentrical aberration 
is correspondingly diminished. The immersion lens, therefore, pre- 
vents the escape of more than half of the rays diverging from a 
podura bead if mounted in Canada balsam, so that the quantity of 
rays transmitted by the water is two and one-fourth greater than 
that by the dry lens, as will be shown. 
