232 
Aperture of Object-glasses. 
and the angle diminished to 98°. Still the rays after the first re- 
fraction take the same direction, d, e. 
Instead of water, let us immerse the lens in Canada balsam. The 
index of this being taken as the glass of front, no refraction will 
occur at the first surface, but the rays will proceed straight in the 
direction d, e, extending the focus to g, and reducing the aperture 
near to 80°. In this demonstration it is needless to complicate the 
question by carrying the rays through the entire combination, as 
we may take them at their final emergence at the longest conjugate 
focus possible, or just within parallelism, d, e, c, must in that case 
be fixed in direction and position for the dry lens, d, e, f, for the 
water lens, and d, e, g, for the balsam lens. 
If we decrease any of the focal distances, c, f, g, from the front 
surface, so as to increase these angles, the conjugate focus must 
become extended correspondingly; and having already taken the 
extreme length, the posterior rays would no longer he parallel, but 
divergent. The effect in this case would be negative, and no image 
be formed in the eye-piece. Now, with a shorter conjugate focus at 
the back, perfect images are obtained ; but the points c,f, g, would be 
more distant, and less aperture the certain consequence. We now 
come to the question of the immersion versus the dry principle. 
This simply means that a film of air shall intervene, either at the 
surface a , b, or elsewhere in the cone of rays through the medium. 
For the sake of simplicity, we will take this film as one of infini- 
tesimal thinness— a case of cover contact in fact. Now, when the 
