NATURAL HISTORY MISCELLANY. 699 
or so nearly so that the MEN would be imperceptible. Moreover, 
the very minute portion of the extraordinary ray which would undergo 
Euro: would deviate more than two mue from the direction of the 
reflected ordinary ray; and so, supposing this prism to form part of a 
p 
binocular arrangement for Me microscope, would be thrown out of the 
eld. 
But the pencils of rays which go to form the image in the body of the 
microscope have a certain angular spread. If, therefore, the axis of the 
central pencil be ae to a given plane, those of the lateral pen- 
cils will be inclined to the e plane. Accordingly if this central axis 
were to be incident on the Pise plane of section at 379, the inciden- 
between limits somewhat larger. Also as the lateral rays of each pen- 
cil are inclined more or less to the axes of the same pencils, the limits of 
maximum and minimum incidence would be more largely reip by this 
circumstance. For low powers we should have to allow for a range of 
incidences goena perhaps eight or nine degrees of ehk For 
very high powers this range would hardly exceed six 
If the ition of the central axis is fixed at 37° 1l, the angle of total 
reflection for the ordinary ray, then the lateral pencils of this ray, whose 
incidences are less than 37° 11’, will be to a certain, but not very con- 
siderable, degree, transmitted. This does not affect the definition of the 
image seen by transmission, but it gives it a slight wc fa to ist 
other in respect to an If, however, the inciden 
axis is made as tiga as 39°, the two images become eiis eq ws in 
brightness. In this case some of the ideni pencil of the extraordinary 
ray Will attain an "i ien of 429, at which point the amount of reflec- 
tion is quite sensible, but this does not materially affect the middle of the 
field, nor is it sufficient to impair, perceptibly, the brilliancy of the image 
Seen by transmitted light. 
is now about three years since the plan of a binocular founded. on the 
applied to in London, and in this country, but no one was found willing to 
attempt the preparation. In the spring of 1869, Professor Rood, of Co- 
lumbia College, kindly lent his aid to the accomplishment of this under- 
nearly the same index of refraction, of which the first surface was placed 
