Mixed Colouring Matters with the Spectrum Microscope. 129 



character may be inferred without reasonable doubt, and confirmed 

 by a more extended examination, 



I have lately found that many interesting facts may be observed 

 by examining the spectra of substances in their natural state, with- 

 out extracting the colouring matter. Frequently they are so opaque 

 that it is requisite to use a very bright light to penetrate through 

 them. I employ, as a condenser, a plano-convex lens about * 7 inch 

 in diameter, and of about the same focal length, over which is 

 placed a conical cap with a round hole a 'little larger than the field 

 of the microscope as seen through the slit in the spectrum eye- 

 piece, at such a distance from the lens within the focal point that 

 the image of the sun illuminates the entire hole. By placing the 

 object on a piece of thin glass, and bringing this hole close up to it, 

 sufiicient light may be driven through a comparatively opaque 

 object, and none which has not penetrated through it can pass up 

 the microscope, even when the specimen is small. In this manner 

 the petals of flowers may be examined direct ; and if the colour is 

 too pale with one petal, two or more may be pressed flat, one over 

 the other. We may thus readily see two well-marked absorption- 

 bands in the blue in the light transmitted through the petals of 

 several species of Brassiea, of Helianthemum vulgare, Geum urha- 

 num, and several other yellow flowers ; but in the case of many others, 

 as Viola lutea or Ranunculus hulbosus, though similar bands are seen 

 in exactly the same part of the spectrum, they are so much more 

 faint on account of general absorption at the whole of the blue end, 

 that there must certainly be a second yellow colour present. Some 

 yellow flowers, like Chelidonium niajus, give weU-marked absorp- 

 tion-bands in such a diflerent position as clearly to show that they 

 are due to another substance ; but on the whole the spectra of very 

 many can be readily explained only by supj)osiug that they are 

 coloured by a very variable mixture of at least two yellow sub- 

 stances. The facts seen in the case of the spectra of various Algai 

 are,- however, so much more striking, that it appears to me that 

 I could not possibly choose a better illustration, and that a some- 

 what detailed description will serve well to explain the method which 

 I think should be followed in such inquiries. 



The following woodcut shows the spectra of the light trans- 

 mitted by the various plants themselves, so far as relates to the 

 colouring matters soluble in water. I have left out all such absorp- 

 tion as is due to chlorophyll and xanthophyll, for the sake of sim- 

 plicity. I may here remark that it is most important to study the 

 spectra of the plants themselves, since in some cases the colouring 

 matters are decomposed so rapidly in solution that they are almost 

 or entirely lost, and new absorption-bands are developed, due to 

 products which do not occur in the living plants. 



