8PECTRO-MICR08COPT. 127 



of an inch, or more, on one side, and not above - 4 ^jth on 

 the other ; and then the effect of different thicknesses can 

 easily be ascertained. 



" Fortunately, the various modifications of the colouring 

 matter of blood yield such well-marked and characteristic 

 spectra, that there are few subjects to which the spectrum- 

 microscope can be applied with greater advantage than the 

 detection of blood-stains, even when perfectly dry. For 

 this purpose condensed light may be used, provided a 

 sufficiently bright light be thrown on the object by means 

 of a parabolic reflector or bull's-eye condenser. A speck 

 of blood on white paper shows the spectrum very well, 

 provided it be fresh, and the colour be neither too dark 

 nor too light, and the thickness of the colouring matter 

 neither too great nor too little. A mere atom, invisible to 

 the naked eye, which would not weigh above the ioo&oooth 

 of a grain, is then sufficient to show the characteristic 

 absorption-bands. They are, however, far better seen in 

 solution. About T&ffth of a grain of liquid blood, in a cell 

 of ^th of an inch in diameter, and an inch long, gives 

 a spectrum as well marked as could be desired. In 

 exhibiting the instrument to a number of persons at a 

 meeting, I have found that no object is more convenient, 

 or excites more attention, than one in which a number of 

 cells are fixed in a line, side by side, containing a solution 

 of various red-colouring matters. In one I mount blood, 

 which gives two well-marked absorption-bands in the 

 green ; in another magenta, which gives only one distinct 

 band in the green ; and in another I place the juice of 

 some red-coloured fruit, which shows no well-defined 

 absorption-band. Keeping a larger cell containing blood 

 on the stage attached to the eye-piece, these three objects 

 can be passed one after another in front of the object-glass, 

 and the total difference between the spectrum of blood and 

 that of either fruit-juice or magenta, and the perfect iden- 

 tity of the spectra when both are blood, can be seen at 

 a glance. By holding coloured glasses, which cut off the 

 red, but allow the green rays to pass, we can readily show 

 how the presence of any foreign colouring-matter, which 

 entirely alters the general colour, might not in any degree 

 disguise the characteristic part of the spectrum ; and by 



