OPTICAL PROPERTIES OF BLOOD. 



243 



way. In these observations but a small portion of the spectrum is brought 

 into the field of view. The eyepiece in the spectroscope is therefore fur- 

 nished with a screen which can be opened or narrowed at will and sym- 

 metrically, that is from both sides, so as to expose some definite small por- 

 tion of the spectrum. The instrument should be so constructed as to 

 permit any desired portion of the spectrum to be quickly and accurately 

 brought into the field. 



In place of using a simple cell it is much better in practice to employ a 

 cell with so-called Schulz glass prism.' With the simple cell the meniscus 



FIG. 1 8. FIG. 19. 



Absorption cell and Schulz glass prism as used in quantitative analysis by 

 absorption. The position of the prism is shown in Fig. 19. 



formed at the top of the liquid projects a broad dark band across the field 

 horizontally which makes the comparison of the upper and lower spectra 

 very difficult. With the cell furnished with a Schulz prism this difficulty 

 is largely overcome, but the details of the arrangement can not be explained 

 here. They will be easily understood by use of the instrument. When the 

 Schulz prism is employed light enters the upper slit through i.i cm. of 

 solution and the lower slit through o.i cm. of solution and the i.o cm. of 

 the clear glass prism. 



Name of Substance. 



