CH. IX] 



USE OF PROJECTION MICROSCOPE 



253 



385. Kind and color of paper for the masks. The best paper 

 to use is one that allows only a moderate amount of light to pass, 

 and that cuts out the green-blue end of the spectrum. 



The color found best for this is an aqueous solution of the 

 microscopic stain known as "Orange G." For the quality of 

 paper, a white linen bond paper of moderate weight is used. It is 

 stained by soaking it a few minutes (10-30) in a saturated aqueous 

 solution of the "Orange G." It is then hung up to dry. 



FIG. 143. SLIDE OF SERIAL SECTIONS WITH MASK. 



The sections to be demonstrated are left uncovered. 

 Sus (Sus scrofa, the pig). 



Ser. ii This shows that the slide is from the 1 1 th series of pig embryos. 

 si 60. The 6oth slide of series 1 1 . 



Sec ij/J- This indicates that the sections of this embryo were cut 15 microns 



(.015 mm., .00058 in.) thick. ^i" 



i goo The year in which the series was prepared. fit* 



ii 60 At the left; series 1 1, slide 60. K* 



& 

 Paper thus colored allows a moderate amount of light to pass, 



and allows practically all of the long waves of reel and infra-red to 

 pass, so that it will not burn very quickly in the focus of the con- 

 denser. If black paper were used it would burn almost instantly 

 in the focus. Of the many yellows and oranges tried for masks the 

 "Orange G" proved most satisfactory. 



386. How to employ the masks. The paper is cut of the 

 right size for the slide and then square or round holes arc made in 

 it to give a clear field for the different objects to be shown on the 

 slide, then it is pasted on the cover-glass (fig. 143). It is put on 



