100 NATURAL HISTORY MISCELLANY. 
parallel to the terminal plane of the calcite. It was thought that the 
very nearly equal and opposite refractions thus suffered by the ray would 
suffice to prevent sensible esee and this is nearly true. But the 
unequal dispersive power the two substances makes itself slightly 
manifest when the Pichina used are low; though this defect disappears 
in the case for which the instrument is intended — that is with high pow- 
ers. Nevertheless, it has been thought best in new constructions now 
preparing, to give such an obliquity to the terminal plane of the ca cite 
that the reflected ray may be incident upon it perpendicularly, and to 
modify correspondingly the flint glass prism. On the whole it appears to 
be the plane of section an inclination of about 38° in- 
stead of 39°. Indeed it would appear that, for low powers, the lower 
S 
case, there is a larger reflection of the voies ray, which is greatly 
vg ced by a very small change ín the angle of incidence. For this rea- 
son it is convenient to have the system of prisms so cuisse. that it can 
sive a slight rotation about an axis perpendicular to the plane of re- 
flection, and to adjust it to the position most satisfactory with the power 
employed. 
The annexed figure (140) will serve to give an idea of the form of con- 
struction now employed. ABCD i section, parallel to one of the lat- 
Fig. 140. Ae faces, of a calcite prism, origi- 
ki u nally rectangular, of which the optic 
iii B F axis is parallel to the section, and to 
T ee PA [I> : the sides AB and DC. This prism is 
divided by a plane perpenre iim 
ABCD, making an angle of 38° wit 
y AB and 52? with AD. Also, the face, 
E ae inclined 14° to the original face 
igo" | 10. f the rectangular prism, is made to 
ed pee e that face. The prism, when 
completed, sliootd have its lower face 
square, and the side of the square which is equal to DC, should be six- 
tenths of an inch. The remaining dimensions will be determined by this, 
and 2 d which 5 wi be three-twentieths of an inch. The surfaces 
of section, BE, may be brought very near to each other. In the con- 
icon actually pets they have been separated only by a single 
Esso of tinfoil, introduced at each of the angles. 
ism, FGH, is of flint glass with a refracting index as high as 1.56 
or higher. It is isosceles, having an obtuse angle of 92? at F, the acute 
angles being equal and each 44°. The side, FH, being parallel to BC, à 
the second, reflected by BE, passes perpendicularly through the two sur- 
and FH, is a second time reflected by GH, and finally emerges 
