158 Transactions of the Royal Microscopical Society. 
we then only see the object at an angle the same as the incline ; 
therefore for objects in balsam 45° would be preferable. 
In using slides with these prismatic or inclined mounts, it must 
be borne in mind that nearly every object lies under a different 
thickness of glass, according to the distance from the keen edge; 
therefore, having selected a suitable one, the objective is to be care- 
fully adjusted to give proper definition, and a thickness of glass 
over the scale may probably be found that will best suit its correc- 
tion. For dry mountings, the object-glass must be used dry, as 
water would run in and spoil the object. If this is in balsam, of 
course the immersion system can be employed. 
In the dry slide, a direct illuminator is used — such as the 
ordinary achromatic condenser ; for, as shown by the diagram, 
axial light becomes that of excessive obliquity on reaching the 
object. At first sight, it might be urged that colour would arise 
from this extreme refraction or deviation of the axial ray of the 
object-glass ; but such is not the case, because distance must be an 
element in chromatic dispersion or separation ; and as the object is 
on the refracting surface, the outline is as free fiom colour as if 
seen through the usual covering glass. Nor is there any difference 
in the appearance, whether the object is adherent to the face of 
either the upper or lower prism.* 
* As there is no chromatic appearance, dense flint glass might be used for the 
prisms, as this medium having a higher refractive power, more acute angles 
would give the required obliquity of vision. 
