104 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
(2) The image of the drawing board must reach the eye with the 
least possible loss of intensity and coaxial with the microscopic image. 
(3) There must be an arrangement by which the relation of the in- 
tensities of these two images can be changed within sufficiently wide 
limits ; and this arrangement, as in Bernhard’s apparatus,* must allow of 
a change not only of the apparent brightness of the plane of the drawing 
but also of the intensity of the microscopic image. 
(4) The apparatus must be adjustable in height and capable of being 
centered in its horizontal plane. 
(5) It must be possible to easily separate the apparatus from the 
eye-piece and replace it again in its original position at will. 
(6) The image of the plane of the drawing and the image of the 
microscopic object projected on it must be seen with the apparatus with- 
out any distortion. 
The author describes the latest drawing apparatus offered by the firm 
of Zeiss, and shows to what extent it fulfils the above conditions. 
As regards conditions 1 and 2, the author comes to the conclusion, as 
the result of numerous experiments which he has made, that the well- 
known method of Schroder, Govi, and others, which consists in the use 
of a glass plate with a thin metallic deposit on its surface, does not 
sufficiently correspond to the requirements, since the light passing 
through the metallic layer is too much weakened. He accordingly 
adopts instead of this essentially the arrangement of the original Abbe 
camera, viz. two rectangular prisms with the hypothenuses cemented 
together, of which one is silvered with a small portion of the deposit in 
the centre scratched away, and with these a second mirror A (fig. 12) 
for transmitting the image of the plane of the drawing to this prism. 
But since one and the same prism, with i determined opening in its 
silver deposit, cannot suffice for all purposes and changes of magnifica- 
tion, an arrangement is added by which the prism P (fig. 13), with its 
fastening, can be easily taken out of the apparatus and replaced by 
another with an opening of different size. 
With respect to condition 3, the author has sought to render the 
methods adopted by Abbe and Bernhard f less cumbrous by substituting 
for the discs, with their series of smoked glasses, an arrangement of two 
smoked glass wedges, after the principle of Babinet’s quartz-wedge com- 
pensator, one wedge being made to move over the other, so as to form a 
jDlate of continuously varying thickness. By such an arrangement the 
problem in question meets with its complete solution, but for the appa- 
ratus in question the method was abandoned, since it raised the price of 
the instrument too considerably. Instead, a modified form of the 
Bernhard and Winkel arrangement was adopted. The smoked glass 
plates were set in the cylindrical wall of a small cap R (see figs. 12 and 13), 
which was simply placed over the prism. Each smoked glass can be in 
turn interposed in the path of the rays by turning the cap on its upper 
edge until a small pin engages in a corresponding small hole on the 
lower edge of the cylinder. In the cap are five smoked glasses of 
different strengths, while the sixth hole is left empty. 
For diminishing the brightness of the image a disc B, as in Bern- 
hard’s apparatus, with four smoked glasses and a vacant space, is inter- 
posed between the prism and the eye-piece. 
* See this Journal, 1892, p. 263. 
t Loc. cit. 
