244 The Philippine Journal of Science ms 
which in every good micrographic camera is close to the ground 
glass and plate. The necessary adjustments are made to bring 
the image of the object near the shadow of the screen, and 
the exposure is made. Then the plate holder is removed; a 
second screen is fixed to the diaphragm to block out the re- 
mainder of the field, its edge just touching the first screen, 
after which the first one is removed. The micrometer slide 
is then substituted for the object slide on the stage of the 
microscope and adjusted to throw its image on the unexposed 
portion of the plate. A second exposure of the plate is then 
made of the same duration as the first. 
Pieces of black paper with straight edges serve for screens 
and are easily fixed in place by pieces of adhesive tape. Sheets 
of black metal would do as well or better, and clips might be 
provided for holding them in place. Or a pair of diaphragms 
with complementary openings might be employed in place of 
the single diaphragm with its circular opening. However, the 
use of adjustable screens has the advantage of permitting varia- 
tion of the position of the scale to suit the size of the object 
image. 
To facilitate the adjustment of the micrometer slide there 
may be gummed to its upper side two strips of black paper; 
one across the slide to block off all the space beyond one end 
of the scale, and the other lengthwise to block off as much of 
the scale lines as is superfluous, leaving exposed only a short 
portion of each of the lines which mark the smaller divisions. 
The slide thus blocked may be very quickly brought into position 
on the microscope stage and afterward brought into focus. 
CONCENTRATION OF GLYCERIN HASTENED BY THE VACUUM PUMP 
In his most admirable presentation of the glycerin and 
Venetian-turpentine methods of making microscopical prepara- 
tions, Chamberlain 2 directs that the material, properly prepared, 
be put in 10 per cent glycerin (1 part glycerin and 9 parts 
water) and the water allowed “to evaporate gradually in a place 
as free from dust as possible.” He specifically states that : 
It is not necessary to use an exsiccator. Merely put the glycerin into 
shallow dishes, and leave it exposed to the air, but protected from dust. 
If the material is in Petri dishes or other dishes with a large surface, 3 or 
4 days will be sufficient. This process should not be hastened by warming. 
2 Chamberlain, Chas. J., Methods in Plant Histology, 3d revised edition, 
Chicago (1915) 93 and 98. 
