244 The Philippine Journal of Science i9is 



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. 



5 Chamberlain, Chas. J., Methods in Plant Histology, 3d revised edition, 

 Chicago (1915) 93 and 98. 



