
328 THE NORTHERN MICROSCOPIST. 


when, in estimating apertures, a stratum of air exists between the 
object and the front lens of the objective, which is, therefore, called 
a dry objective. We have now to consider the case of zmmersion 
objectives, where the light from the object must traverse a film of 
water or oil interposed between the front surface of the lens and the 
cover-glass, to the under surface of which the object must be adherent, 
if dry, or otherwise mounted in a medium of equal refractive index 
to that of the immersion fluid; and in doing so we shall perceive 
how an immersion lens can have a greater aperture than 180° in air, 
or a greater numerical aperture than 1.0, which is the same thing. 
As a preliminary, I must remind you of certain principles in- 
volved in the behaviour of rays of light passing from air into 
media of greater density, such as water and oil. In the diagram, 
Fig. 38, let A O be a ray of 
light falling obliquely upon 
surface of water at O, and 
refracted in the direction 
of C, or nearer to the axzs 
of incidence B D than the 
original direction of the ray: 
then the sine A B of the 
angle of incidence I will be 
to the sine C D of the angle 
of refraction R as 1.33 to 
1.0, and this proportion 
will be maintained for every 
obliquity of the incident 
ray. The number 1.33 is, 
therefore, the co-efficient of 
refraction in water, or the 
index of refraction for that 
Fig. 38. medium. In like manner 
the index of refraction for crown glass is 1.52, for cedar-wood oil 1.51, 
for Canada balsam 1.53, and for bisulphide of carbon 1.68. Now 
as every ray of light falling upon the point O, within the angle of 
incidence I, must, in passing into water, be refracted within the 
angle of refraction R, it follows that the smaller sine C D in water 
must receive all the light transmitted through the greater sine A B 
in air; and that equal plane angles in air and water must, under 
similar conditions, receive amounts of light in the respective pro- 
portions of 1.0 and 1.33. Now let the line C D represent one- 
half of the diameter of the front surface of a water-immersion 
objective having its focal point in O, where an object is placed in 
contact with the immersion medium and illuminated by the semi- 
angle of light B O A: then, as the amount of light falling upon 
C D in water is equal to the amount falling upon A B in air, it 



