1877.] 



Properties of Doubly Refracting Crystals. 



385 



in its Journal*. After the reading of this paper I said that it appeared 

 to me probable that the same principle might be applied with advantage 

 to the determ in ation of the index of retraction of minerals. The chief 

 question was how to make the requisite measurements by means of such 

 an addition to an ordinary microscope as would not in any way interfere 

 with its general use for other purposes. This I accomplished by fixing 

 a graduated scale to the body of the microscope and a vernier to the 

 supporting arm, so that the position of the focal point can be read off to 

 within about °^ 30 i^'h- I described this arrangement and 



pointed out its value in connexion with mineralogy at a meeting of the 

 Mineralogieal Society last March, and an account of it was published in 

 the Journal of the Society*. I have since learned that a very similar 

 addition was made to a microscope in Professor Clifton's laboratory at 

 Oxford some eight years ago, and used for the measurement of the index 

 of refraction of glass, but no account of it was ever published. 



When I came to study the index of refraction of doubly refracting 

 minerals I was very soon struck with the fact that, instead of seeing at 

 one focus the two systems of lines at right angles to each other, they 

 were sometimes quite invisible, or one set was seen at one focus, and the 

 other at a very different, as though they had been ruled on the two oppo- 

 site sides of a piece of glass. These curious phenomena were exhibited 

 at the soiree of the Eoyal Society on the 25th of April last, and Pro- 

 fessor Stokes immediately examined the question theoretically, and found 

 that they could be explained by, and might have been predicted from, 

 the known laws of double refraction, though apparently no one had ever 

 studied them, either theoretically or practically. ~\\~e therefore de- 

 cided to investigate the problem independently.! I was to make the 

 practical observations, and he to give the theoretical explanations, the 

 results being kept separate, but communicated conjointly to the Eoyal 

 Society. 



There was no difficulty whatever in proving that the general pheno- 

 mena agreed with the results of theory : but when I came to compare 

 the numerical results of observation and calculation I found that many 

 unexpected difficulties had to be contended with. The sections or 

 minerals, originally made for an entirely different purpose, were in many 

 cases not cut with sufficient accuracy, or were too full of flaws. It was 

 some time before I could obtain suitable specimens of some minerals, 

 and it was necessary to recut nearly all my original preparations. I also 

 found that there were several unforeseen sources of error, and that it was 

 requisite to repeat nearly all my measurements. Much to my regret, 

 I therefore found it necessary to publish along with Professor Stokes's- 

 paper, either a large number of indices known to be inaccurate and not 

 suitable for comparison with theory, or to publish this present pre- 



* 1876, vol. xri. p. 294. 



t 1377, vol. i. p. 97. 



2d 2 



