104 The Microscope. 



D line, at which point the color is a bright yellow. So it is with 

 the other colors. The blue becomes very dark at G, and is com- 

 monly called indigo. 



If a little fresh arterial blood be now placed upon a slide 

 covered with a thin cover-glass and placed upon the stage, in 

 addition to the narrow Fraunhofer lines will be seen two broad 

 dark bands, one in the yellow and the other in the green. It 

 will also be noticed that from a little to the left of F, the light 

 is entirely gone from the blue end of the spectrum. These dark 

 bands are produced by the absorption of the colors which cor- 

 respond to their position, as it passes through the object. It will 

 further be noticed that the edges of these absorption bands are 

 not sharp and distinct, but rather nebulous, shading off grad- 

 ually. If a thin solution of blood should be used, or the layer 

 be very thin the lines are correspondingly faint, and if the layer 

 be increased in thickness the bands become darker and wider, 

 without, however, altering their position as regards the centers 

 of the bands. 



If the spectrum, as produced by passing through blood, be 

 compared with that of any other fluid or solid, it may be done 

 by drawing down the milled head which moves the comparing 

 prism. If the light be then reflected upon this prism by the 

 small mirror, two spectra will be seen side by side. The tube, 

 bottle or slide should then be placed upon the small stage, over 

 the central hole, and held in position by the ledges and spring. 



If it be desired to preserve a map of the spectra, it may be 

 done by noting the position of the bands as regards the princi- 

 pal lines, or a spectrum scale may be used, which consists of a 

 polarized plate of quartz, which gives a series of dark lines 

 across the spectrum, with which the bands may be compared. 

 One of the most convenient methods, however, is by using the 

 camera lucida. The cap (K) of the instrument may be re- 

 moved and the camera put in its place ; then by placing the 

 microscope stand in a horizontal position, the position of the 

 lines may be accurately mapped. If used by a lamplight even 

 this method will necessitate the use of a standard scale or some 

 fixed line. For this purpose I use the bright line of sodium 

 vapor which is shown in No. 12 of the plate. This line may be 

 produced by putting a small quantity of common salt on the 



