NATURAL HISTORY MISCELLANY. 101 
at right angles from the face FG. The inclination of o to e is twelve de- 
grees. It would be preferable to make it somewhat less, as this inclina- 
tion allows only a length of body to the microscope of about seven inches. 
By employing in the prism, FGH, glass of higher refracting power, it may 
be made less, and by using calcite for this prism, or in other words, by 
making BCDE and FGH all of a single piece, the same object may be at- 
tained to any desired degree. The objections to this latter plan are two- 
"S The first relates to the difficulty of construction. It is said that 
e Wenham trapezoidal prism of glass is troublesome to make. The 
he would be much increased in the use of such a material as calcite, 
especially when it is necessary to preserve an exactly prescribed relation 
between the faces of the prism and the optic axis. The second objection 
is found in the consideration that, in order to adapt the tubes of the bi- 
nocular to the eyes of different observers, it is necessary to give to one 
of the tubes an angular movement, moving the prism, FGH, at the same 
time, by half the same angular amount, as is done by Mr. Nachet in one 
of his forms of binocular; or to move this tube and prism laterally, as 
Mr. Nachet has also done in another of his forms. This necessity arises 
from the fact that, if the tubes are sufficiently inclined to each other to 
a it 
vide construction, and of greater size than is desirable. 
there is another objection to the crossing of the pencils which is 
i serious. This binocular, as actually constructed, dese when 
ib t 
used with moderate powers, à Sensibly stereo- Fig. 14 
scopic effeet. Nor is it difficult to understand why Wa Py 
it should do so. any stereotomic binocular, 
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seen that if aa'a'! be the axial ray of a converging 
pencil of which bb'b” and cc'c/ are the lateral lim- 
iting rays, and if a transparent reflector, MN, be in- 
 terposed obliquely in the path of this pencil, the angles of incidence of all 
the rays intermediate between a! and b' will be larger than those of the 
rays between a! and c'. Of the reflected rays, therefore, those between 
al!!! and b!!! will be more abundant than those between a!” and c; while 
b a e 
ill be a corresponding deficiency between a!’ and b". Now if all 
