Oblique Vision of Surface Structure. By F. H. Wenham. 157 
the appearance of the structure is the consequence. The closeness 
of the high powers to the covering glass will not permit the slides 
to be tilted, to an extent sufficient to cause any appreciable differ- 
ence in the appearance of the object. 
The following arrangement will produce an extreme obliquity of 
vision obtained with the axial pencil of the object-glass, a, Fig. 2, 
is a slip of glass about rb- wide, ground and polished off to an angle. 
Objects to be mounted, such as diatoms or lepidopterous scales, are 
scraped up with the knife-edge, so as to be distributed thereoD along 
the sloping plane. Those situated near the edge may he viewed with 
the highest powers, as the glass is of course thinner here than any 
cover. The thickness of the remainder of the prismatic slip is of 
no consequence, and it may he of the same gauge as an ordinary 
thin slide. The slip is tacked on to a 3 x 1 slide with a dot of 
balsam or cement. Another similar slip, b, is then pressed endways 
against it, so as to lay the objects flat between the two inclines. 
The lower prism is necessary ; for without it, a deal of offensive 
colour enters the object-glass from the decomposition of the trans- 
mitted light. This is recomposed or neutralized by the under prism, 
which also greatly increases the obliquity of the illuminating ray 
by refracting this to the same angle as that of vision, from the 
deflection of the axial ray of the object-glass. 
The conditions are as follows, a, Fig. 3, is the central ray of 
the object-glass. This ray, after leaving the upper inclined surface, 
is refracted in the direction b. If the surface is inclined near 40°, 
the upper side of the object will he viewed from a small elevation 
of between one and two degrees, according to the refraction of the 
glass ; conversely, an illuminating ray thrown up axially with 
the microscope will strike on the object at the same degree ; both 
the visual and illuminating angles will therefore be similar, as 
represented by line c, c'. 
The degree of inclination of the facets of the prisms for dry 
objects should be less than 40°; for on holding before a flame a 
slide having this angle, and tilting it slightly, the width of the 
junction of the prisms appears as a dark band impervious to light — 
the effect of total reflexion. About 35° is therefore more suitable 
for objects mounted dry. If balsam is run between the inclines, of 
course total reflexion is eliminated, and refraction nearly so, and 
n 2 
