PROPERTIES AND METHODS OF IDENTIFICATION 221 



on the concentration of the solution used and the depth of the layer 

 through which the light passes into the spectroscope and thence to 

 the eye of the observer. This fact, together with the fact that the 

 edges of the absorption bands are not sharp and clear cut like the 

 lines of the solar spectrum, no doubt explains the slight differences 

 between the data given by various observers as to the width of the 

 several absorption bands of carotin and the other carotinoids. In 

 spite of this fact, however, the absorption bands of carotin solutions 

 are sufficiently characteristic to distinguish the pigment sharply from 

 the other carotinoids, at least from lycopin and the xanthophylls and 

 rhodoxanthin. Plate 1, showing a spectrophotograph of the bands 

 of carotin and xanthophyll in alcohol and carbon disulfide, brings 

 out this point very clearly, as well as the diffuse character of the 

 edges of the bands. It may be stated, however, that the bands may 

 be somewhat sharper to the eye than is represented in these photo- 

 graphs. The characteristic feature of the bands of carotin which it 

 is desired to point out is that in alcohol (an identical spectrum is 

 obtained in ether and petroleum ether) the solar line F divides the 

 first band almost exactly into two equal parts. This is a character- 

 istic of the first carotin band which may serve to identify the caro- 

 tin spectrum from that of the other carotinoids. 



For direct spectroscopic observations a spectroscope with too wide 

 a dispersion may fail to show any bands in a carotin solution which 

 exhibits very beautiful bands using a spectroscope with a moderate 

 dispersion of the spectrum. In working with unknown biological 

 material the writer has had better success using an inexpensive spec- 

 troscope with a moderate dispersion whose spectrum field has been 

 standardized, although arbitrarily, first with the sodium flame and 

 then with known solutions of the carotinoids. Such a spectroscope 

 set up in a dark room with a light of high candle power concentrated 

 on the slit of the instrument but screened from the observer, gives 

 excellent results. 



Willstatter and Stoll (1913) give the following measurements for 

 the absorption bands of carotin in solutions containing 5 mg. of pig- 

 ment per liter, using a grating spectroscope. These data correspond 

 with the spectro-photographs shown in Plate 1. 



Carotin in carbon 



Carotin in alcohol (mi) disulfide (mi) 



6mm. 10mm. 10mm. 20mm. 



Band I 492-478 492-476 524 -510 525-508 



Band II 459-446 459-445 489-475 490-474 



