33 2 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



spectra are placed one above the other, and the posi- 

 tion of the absorption bands of one can be determined 

 by the other. By means of the spring s, a thin tube 

 containing a solution can be held over the opening K, 

 so that the spectrum of a substance on the stage of the 

 microscope can be compared with the spectrum of the 

 substance at the side. In some forms of micro-spectro- 

 scopes a contrivance is added for measuring the exact 

 position of the absorption bands. It is inserted into 

 the tube A just at the level of the upper end of the 

 series of prisms. It consists of an arrangement for 

 throwing an image of a fine line on the upper sur- 

 face of the last prism in such a direction that the 

 prism reflects it into the eye of the person looking 

 down the tube. This fine line is seen, therefore, 

 crossing the spectrum. By means of a screw the line 

 can be moved along the spectrum from one end to the 

 other, and made to coincide with anv of its dark 



> 



lines. A micrometer attached to the screw measures 

 the extent of movement of the line. Thus, suppose 

 the screw had to be moved through a distance, 

 measured by 30 on the micrometer, in order to 

 make the line coincide with a dark line of the solar 

 spectrum, and suppose then some substance laid on 

 the stage gave an absorption band, and that the fine 

 line of the micrometer had again to be moved through 

 30 to make it coincide with the absorption band, it 

 would be known that the absorption band coincided 

 with a dark line of the spectrum. 



A micro-spectroscope would readily indicate 

 whether a stain on clothing had been caused by blood 

 or not. The stain simply requires to be cut out of 

 the cloth, and placed in a watch-glass in a few drops 

 of water. The coloured solution obtained, placed in a 

 cell on the stage of a micro-spectroscope, would give 

 the two bands of oxy haemoglobin, even though the 

 drop of blood had been very small. There are various 



