1018] on The Spectroscope in Organic Chemistry 705 



But when we extend this method of investigation to colourless 

 subst^mces, we are dealing with phenomena which lie hidden from 

 the unaided eye, and our investigations are necessarily carried out 

 with the help of photography. 



The instrument employed in the study of absorption spectra con- 

 sists of a spectroscope in which the eye-piece of the telescope is 

 replaced by a camera. The photographic plate is set at such an 

 angle as to bring all the rays emanating from the source of light into 

 focus at its surface after they have passed through the resolving 

 prism, and for this purpose it is necessary that the plate should have 

 a very slight curvature. The prisms and lenses of the apparatus are 

 made of quartz, which, unlike glass, is readily permeable by the ultra- 

 violet rays. The source of light usually employed is that obtained 

 by sparking one of the metals, such as iron, or a combination of 

 metals, such as cadmium alloyed with lead and tin, selected so as to 

 give a large number of lines spread as uniformly as possible over the 

 whole ultra-violet region. The advantage of such a spectrum over a 

 continuous spectrum is that the position of absorption bands, when 

 they occur, can easily be determined with reference to the metallic 

 lines whose wave-lengths and place in the spectrum are known with 

 great accuracy. In using the apparatus a photograph is first taken 

 of the spectrum of the source of light. A layer of the substance to 

 be examined, which, if a solid, must be dissolved in a suitable 

 diactinic solvent, such as alcohol or water, is then interposed between 

 the source of light and the slit of the collimator, and another photo- 

 graph is taken. By comparison of the two photographs it is seen 

 what effect, if any, the substance has had upon the transmission of 

 the light. 



When organic substances are examined in this way it is found 

 that some allow hght to pass freely through them. Others shorten 

 the spectrum by absorbing the rays at the ultra-violet end to a greater 

 or less extent, and are said to show general absorption. Others, again, 

 possess the remarkable property of absorbing rays of a particular 

 wave-length, thereby producing gaps or bands in the spectrum ; 

 these are said to show selective absorption (Fig. 2). 



In studying these phenomena in their relation to the chemical 

 characters of a substance, it is of importance to determine not only 

 the extent of the general absorption and the number and position 

 of the absorption bands, but their degree of persistence, i.e. the 

 range of concentration within which they are exhibited. It is 

 necessary, therefore, to vary the concentration of the solution or 

 the thickness of the layer so as to cover the whole phenomena oi 

 absorption. This is done by simply diluting the solution, or diminish- 

 ing the thickness of layer on the one hand, until the entire spectrum 

 is transmitted ; and on the other hand, by increasing the concentra- 

 tion or the thickness of the layer until no further characteristic 

 absorptive effect is produced. Photographs (Fig. 3) are taken at each 



