1867.] Mr. H. C. Sorby on the Spectrum Microscope. 433 



III. " On a Definite Method of Qualitative Analysis of Animal and 

 Vegetable Colouring-matters by means of the Spectrum Micro- 

 scope." By H. C. Sorby, F.R.S., &c. Received April 10, 1867. 



CONTENTS. 



PAGE 



1. History of the subject .... 433 



2. Description of the Apparatus . 433 



3. Scale of Measurement .... 434 



4. Symbols used to describe Spectra 436 



5. General Eemarks on Absorp- 



tion, &c 438 



6. Methods of Research .... 439 



7. Preparation of Vegetable Colours 439 



8. Examination of the Colours . . 440 



9. Eeagents used 440 



10. Solvents 441 



11. Action of Acids and Alkalies . . 441 



12. General Absorption 441 



13. Fading of Solutions 443 



14. Absorption-bands 444 



PAGE 



15. Action of Sulphite of Soda . .447 



16. Groups A, B, and .... 448 



17. Eeagents used 449 



18. Sej)aration of Colours into 



Groups, «&c. . 449 



19. Order of the Experiments . . . 450 



20. Division into Sub-Groups . . . 451 



21. Individual Colours 452 



22. Mixed Colours 453 



23. Spectra with no Absorption- 



bands 453 



24. Yellow Colours 454 



25. Fading of Group C 455 



26. Conclusion 455 



1. History. 



My attention was first directed to this subject by reading a report of 

 Professor Stokes's very excellent lecture at the Royal Institution, Friday, 

 March 4th, 1864. It immediately occurred to me that a spectroscope 

 might be combined with a microscope, and employed to distinguish coloured 

 minerals in thin sections of rocks and meteorites. I was soon led to exa- 

 mine many other coloured substances, and found that the instrument is 

 more useful in connexion with qualitative analysis, when only very small 

 quantities of material can be obtained. At first I em.ployed the imperfect 

 apparatus described in my Paper in the * Quarterly Journal of Science ' *, 

 but afterwards, along with Mr. Browning, I constructed that described in 

 my Paper in the * Popular Science Review ' f. For general purposes I do 

 not think this could be much improved ; but for chemical testing it is 

 much less fatiguing to use a binocular instrument. There were many 

 difiiculties to contend with, but at length I constructed one which ap- 

 pears to answer all the requirements of the case. 



2. Apparatus. 



I have an ordinary large binocular microscope, and use an object-glass 

 of about three inches focal length, corrected for looking through glass an 

 inch thick ; the lenses being at the top, so as to be as far as possible from 

 the slit. This is placed at the focus, and between it and the lenses, at 

 a distance of about half an inch from them, is a compound prism, composed 

 of a rectangular prism of flint-glass, and two of crown-glass of about 61°, 

 one at each end. This arrangement gives direct vision and a spectrum of 

 the size most suitable for these inquiries, since a wide dispersion often 

 makes the absorption-bands far too indistinct. In order to be able to 

 compare two spectra side by side, a small rectangular prism is fixed over 

 ^ April 1865, vol. ii. p. 198. t Vol. v. p. 66. 



VOL. XV. 2 o 



