ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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followed by that of Nachet, Abbe, and Oberbauser. Nobert’s and many 
others work upon the same principles. 
The following is an explanation of the principle of this drawing 
apparatus: — If the glass plate gl (fig. 29) stands at an angle of 45° 
with the axis of the eye, the rays from the object o (which on their part 
also form an angle of 45° with the glass plate) aro reflected, and 
the picture of the object is seen in a position that also forms a right 
angle with that of the object. If m is the cylinder of the Micro- 
Fig. 23. 
Fig. 30. 
scope, and pp the piece of paper, in this case the eye will sec upon the 
paper at o the picture which is projected by the transparent condition of 
the glass plate <jl. In this case we say that the picture is projected ; 
but if we place a prism p (fig. 30) upon the same level with the glass 
plate gl, and o is the object under the Microscope standing in a 
vertical position m, the glass gl forming an angle of 45° with the axis 
of the eye and standing upright over the ocular, we will then see the 
picture at o' upon pp. Meanwhile, the projected picture of the object 
may also be seen in the horizon. Upon this basis rests the above- 
mentioned drawing prism, of which Nachet’s is the one most commonly 
in use in Europe after Zeiss’s. In this apparatus a prism is employed 
in place of the glass plate, while a second grapho-prism moves upon its 
own axis so as to bring the reflecting surfaces at different angles. The 
purpose of a drawing prism is obvious as soon as it has been placed 
upon the ocular and adjusted. 
Oberhauser’s camera is somewhat more complicated than the others. 
For this reason I will give more in particular the details in regard to 
its practical application. The ascending rays from the objective are 
totally reflected through the large prism d into the horizontal arm A. 
If the ocular is placed in a horizontal position B it directs the rays into 
the small prism C upon the figure of an angle of 45° if focused in 
the right position, where it is again reflected at a right angle into the 
observing eye. 
Oberhauser’s camera is very much liked for this reason : it does not 
create a disturbance or a confusion by the reflection of the picture at 
