112 SUMMARY OF CURRENT RESEARCHES RELATING TO 
shutter by passing the strip of taffeta, as seen in fig. 9, over four 
rollers in such a way that a circular aperture cut in one end of the 
taffeta passes directly in front of the plate at exactly the same time 
that a rectangular opening cut in the other end of the taffeta passes 
between the source of light and the object. The prism arrangement 
remains the same as above described, and must be released by a device 
similar to the mechanism by which the propelling of the taffeta strip 
is effected. 
In fig. 9 m n represents the slit for exposure, a b the circular aper- 
ture covered with a red gelatin pellicle, cd the rectangular aperture, 
allowing light to enter during exposure, w , x,y,z , the rollers upon which 
the taffeta strips are propelled. 
On the Cooling of Projection Preparations.* — Dr. 0. Zoth points 
out the difficulties attending projection by means of very intense sources 
of light, when the object is sensitive to a rise in temperature, as in the 
case of living animals or plants. The usual method for absorbing the 
heat rays consists in the use of troughs of water and concentrated solu- 
tions of alum, or of plates of alum. The author gives the following 
table showing the effect of different thicknesses of water in the absorption 
of the heat rays in the case of the Locatelli lamp : 
Layer of water Percentage of 
in mm. heat-rays transmitted. 
1 .. 19*3 
5 .. 9-1 
10 .. 7-7 
50 .. 2*4 
100 1-3 
150 .. 0*7 
200 .. 0-7 
The table shows that no very great advantage is obtained by increasing 
the thickness of water beyond 50 mm. 
The best and simplest heat-absorption apparatus consists of a trough 
about 5 cm. thick, which is filled with distilled water and placed in the 
path of the illuminating pencil. An experiment made in order to test 
the utility of such an absorption-cooler showed, in the case of a Duboscq 
projection lantern, that whereas the rise in temperature without the 
absorption apparatus was as much as 35° within two minutes, after the 
cooler had been introduced the rise in five minutes only amounted to 
10° and the temperature then remained constant at 30° to 35°. The 
absorption-cooler, however, does not suffice for all cases ; even pre- 
parations mounted in resin which are to be observed under very high 
powers begin to shrink, and the resin softens as soon as the focus of 
the Abbe illuminating apparatus is brought near to the preparation. 
For such cases, therefore, the author recommends, besides the cooling 
by absorption of the heat rays, the direct cooling of the preparation 
by conduction. 
The apparatus which he has designed for this purpose consists, as 
seen in fig. 10, of a brass plate 6*25 mm. thick, 70 mm. long, and 
40 mm. broad, in the middle of which is bored a circular hole of 18 mm. 
* Zeitschr. f. wiss. Mikr., x. (1893) pp. 1526 (1 fig.). 
