22 On the Optical Advantages — [ Menthiy Microscopical 
be thankful,” but let us prosecute the search for higher achievements 
still. 
Reasons for the Construction of the Tables. 
It often happens that even opticians, as well as amateurs, are 
at a loss to draw accurately on a large scale (necessary for useful 
inquiry) the path of a ray of light traversing successively media 
in contact, possessing different refractive properties and densities, 
and at different angles of incidence measured from the normal to 
the surface at the points of penetration or emergence. Frequently 
the possession of such tables would have greatly assisted the writer 
in original research; and he therefore supposes others would find 
them equally useful. They require to be calculated for refractions 
from one substance into another, and not merely for a ray passing 
from air into a more refracting medium. 
Thus though the indices of refraction are for 
Mean Rays of the Solar Spectrum. 
Platewlass® 2.0 3. «« = aeono 
Canada balsam .. .. wp =1°532 
Water, 2s Sonckas: 90. eel oo 
&e. &e. 
the path of the ray of light cannot be shown on paper very readily, 
except as passing from air into the substance whose refractive index 
is known, and then only by taking the sines of the angles of inci- 
dence and refraction, as «7:1. But the practical difficulty is greatly 
increased when the ray 18 passing from one medium into another of 
considerable deflecting power, as from glass into water; for the 
mutual or intermediate index of refraction is then considerably 
modified. Having carefully calculated these quantities, I may state 
that the actual index of refraction for a ray of light passing 
4 a ae 
From Canada balsam into plate glass is 0-9791 
1 
» Crown glass », flint ” 0-83705 
» Canada balsam ,, flint glass ,, oe 
1 
4 Pen pentine Son’ idtiyat 2 Siilgemsnae 
The deviations of the ray entirely depend upon these quantities 
peculiar to the refraction between the contiguous media. For 
example— 
Suppose a brilliant particle be immersed in Canada balsam, and 
that a ray of light emanating from it traverses successively, the 
balsam, the glass cover, a layer of water, and then one of glass (the 
front lens of the objective), are there any optical advantages gained 
