ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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effect. The brass carbon-holders z z , with their clamps v v , are insulated 
by porcelain brackets w w. The current enters and leaves by binding 
screws, not visible in the figure, at the side of z z. Carbons of various 
diameters corresponding to the various current intensities can be used. 
Current intensity should lie between 5 and 20 amperes, the highest 
value being suitable for the greatest possible requirements. If an in- 
stallation current is too powerful, a resistance apparatus should be intro- 
duced into the circuit. The author recommends that of Schuckert & Co. 
II. The Projection of Search- Slides . — It is frequently desirable to 
search systematically a section of a large organ. Such a section may be 
8 or 10 cm. in diameter. Professor Behrens projects it by an ordinary 
photographic objective, and obtains a magnification of 25 diameters : 
thus the image may be some 2 metres in diameter ; an iris diaphragm 
stops off any parts not wanted. 
Fig. 157 . 
The arrangement is seen in fig. 158. The prismatic guides pp, with 
the diapositive bearer and the objective board O, can be removed en masse. 
D is a thick, circular, blackened, perpendicular brass plate of 22 ccm. 
diameter, perforated for a 10 cm. iris diaphragm operated by a lever h. 
The metal clips K K are secured by screws, which not only prevent 
falling out, but permit adaptation to slides of different thicknesses. If 
a slide is smaller than the opening, a piece of plate glass must be placed 
as a backing. The opening is protected on the reverse side by a plate 
of mica to prevent injury to the slides from overheating, and Professor 
Hermann (to whose suggestion the entire appiratus is due) found this 
mica plate so satisfactory that an exposure of 10 minutes to a 20-ampere 
current through a Schuckert lamp produced no melting of the Canada 
balsam. The diaphragm should be placed at such a distance from the 
condenser that the converging light-cone is but slightly larger than the 
iris. If a higher than 25 diameters magnification of any part be desired, 
1899 2 X 
