Aperture of Object-glasses . 233 
refractive index of films of air, that of water, and again of balsam or 
glass, are dragged in as an attempt at a wonld-be scientific demon- 
stration, I denounce it as a muddle and downright nonsense. Thin 
parallel plates of either of these intervening in the cone of rays will 
in no way alter the angle, as they always emerge in their original 
direction. A degree of lateral displacement according to the thick- 
ness is the only result ; and taking, as I have here done, the con- 
stantly occurring case of an infinitesimally thin film of air (as when 
the object-glass is in contact with the cover), neither calculation nor 
diagram will show a perceptible difference in the angle, which is the 
same whether the air is there or not, wet or dry. 
I may put forth this statement without being deemed arrogant 
by those conversant with optics. The ground is safe, and anyone 
that ignores or denies such a definite principle must expect to 
forfeit all respect for his optical knowledge. 
Let us now proceed to the indefinite, wherein the best of us 
are still at fault. I allude to the present unsatisfactory mode of 
measuring the angle of aperture of object-glasses by means of the 
well-known “ sector.” For anything beyond 90° this is by no 
means trustworthy. The microscope body with the objective is 
swung round from a pin near the focal point as a centre, and the 
measure of the angle of the sweep is indicated by the light from a 
flame some few feet off bisecting the field of view in the eye-piece. 
With large apertures a definite bisection cannot be obtained, and 
the limit has to be guessed at by the position at which the light 
seems to vanish. A border of diffused light frequently exists, and 
is thus often mistaken for aperture. Under the most favourable 
circumstances this mode of measuring extreme apertures is so faulty 
that when statements are brought forth such as 172° “ and a half” 
I cannot help smiling at its absurdity, as a mere piece of brag. 
This inaccuracy has long been known, and attempts have been 
made from time to time to provide a remedy. I consider that the 
most correct of all is that proposed by Professor Eobinson, which 
consists in passing the parallel rays of the sun through the back 
of the objective, and then by means of a white screen in a dark 
room or camera intercepting the rays as a disk of light. The angle 
taken from the diameter of this to the focal point will give the true 
aperture. Any extraneous or diffused light is either dispersed, or 
can be detected from its different appearance on the screen. The 
reason why this plan is not generally used is on account of the 
trouble attendant upon the arrangement. 
Effective aperture of course means the production of a distinct 
image up to the extreme marginal ray. If this image at last becomes 
so distorted as scarcely to merit the name (such is frequently the 
case), it may as well be stopped altogether, as the object-glass will 
perform better with a less angle. 
