348 On the Spectra of Pigments. 



ON THE SPECTRA OF PIGMENTS. 



BY HENRY J. SLACK, F.G.S., 

 Member of the Council of the Microscopical Society of London. 



When light reaches us by reflection from a coloured object 

 illuminated by white light, the effect upon our eye. results from 

 the rays which remain in the spectrum after the object has 

 removed by absorption other rays which are complementary to 

 its own colour, and which would make white light if added to 

 that colour. In like manner, coloured transparent or trans- 

 lucent bodies, on which white light falls, absorb certain rays 

 and transmit others — those which they absorb and those which 

 they transmit being together equal to white light. If therefore 

 we view through a spectroscope that portion of white light 

 which is transmitted or reflected by a coloured object, the 

 character of the spectrum will indicate the exact nature of the 

 absorption that has occurred. 



The application of the spectroscope to the microscope just 

 made, in a new pattern, by Mr. Browning, and figured in the 

 last number of the Intellectual Observer, is exceedingly 

 convenient for such experiments, and I have made several, both 

 with flowers and pigments. It is the latter only that will now 

 be described. The most interesting pigment spectrum I have 

 as yet seen is that of cobalt. A little patch of the water- 

 colour paint so named is laid in flat tint on a piece of white 

 paper, illuminated by a bull's-eye, lieberkuhn, or side reflector, 

 and viewed through the micro-spectroscope, when two bands 

 are seen in the green, much like those of the blood spectrum, 

 and a third line in the red. An inch or two-thirds is a con- 

 venient power for these experiments ; and it will be remarked 

 that, in the cobalt spectrum, the bands in the green look green, 

 and that in the red looks red, both being of darker tint. If a 

 minute spot of blood is placed upon a piece of white paper, and 

 a spot of cobalt immediately above or below it, both will come 

 into the field at once, with an inch or two-thirds, and the 

 resemblances or differences between the two spectra can be 

 conveniently observed. 



It is interesting to compare the spectrum of ultra-marine 

 with that of cobalt. With care, a cloudiness is seen affecting 

 the green somewhat unequally, but no distinct band in the 

 spectrum of the former pigment. 



When a little patch of vermillion is seen through the 

 micro-spectroscope, the purple is stopped out, and the green 

 converted into a dark greenish grey. 



