The Microscope. 49 



spectra can be seen side by side. Whilst one shows absorption lines 

 the other shows the Frauenhofer's lines, and admits of locating any 

 band or bands. We call these spectra, comparing spectra, Ver- 

 gleichresspectra. Thus, if we desire to ascertain whether two 

 substances are alike, especially fluids, we obtain two spectra, a fact 

 which will answer the question at once. 



Another mode is to fix the position of any spectram we employ 

 in the interior of the tube of observation, i. e., the one that carries the 

 eye-piece, either a fine wire, a bright line or cross, which may be 

 moved along the spectrum and the scale. Such an arrangement may 

 be found in Browning Soby's bright line micro-spectroscope. 



At some future time I shall enter more in detail, and until then 

 I must also defer the discussion and description of the difracting 

 spectroscope. 



My readers probably all know what happens when a ray of 

 light passes through a prism. Such a ray is decomposed, filtered as 

 it were, into its ultimate component parts, consisting of various 

 colored rays spread out fan-like, forming the spectrum. It contains 

 all tints of the rainbow in regular succession, from red to orange, 

 yellow, green, blue and violet. Whence come these beautiful tints, 

 seeing that the rays passing into the prism were white? 



Paint all these colors upon a disc and "move it rapidly by some 

 mechanical contrivance. The disc will appear white. So, when all 

 these tints in their swift motion strike the retina in the same unit of 

 time, the impression will be that of white light. But when the 

 filtering process begins by means of the prishi, some of the tints 

 pass faster through it than others, and although the difference in 

 the passage must be calculated by many billions of oscillations per 

 second, it is sufficient for the retina to be impressed with the various 

 tints and to bring it to the consciousness of the brain. 



There is another change witnessed at the same time: in 

 passing through the prism the tints change their straight coui'se and, 

 deviating to the right and to the left, are dispersed. We call this 

 the refraction of rays. 



The violet part of the spectrum is far more widely bent out of 

 its course than the red part. This deflection and greater refrangi- 

 bility of the violet rays depends upon the constitution and nature of 

 light itself, whose waves are propagated through space by the wave- 

 like motion of a subtle, highly elastic fluid known as " the luminous 

 ether." The light waves diflPer in length ; the longest form the 

 extreme red part of the visible spectrum, the shortest those of the 



