Mr. Potter 07i Conical Refraction. 347 



point on the first surface miglit be, as nearly as possible, in 

 the direction of the cusp-ray. The exact adjustment to this 

 direction was made by subsequent trial." After relating some 

 preliminary experiments, he says again, " In all these experi- 

 ments the emergent rays were received directly by the eye 

 placed close to the aperture on the second surface. It was 

 obviously desirable, however, to receive them on a screen, and 

 thus to observe the section of the cone at different distances 

 from its summit. After some trials, I effected this with the 

 sun's light, the light of a lamp being too weak for the pur- 

 pose. The emergent cone being made to fall on a screen of 

 roughened glass, I was enabled to observe its sections at va- 

 rious distances, and therefore with all the advantages of en- 

 largement. The light was sufficiently bright, and the ap- 

 pearance distinct, when the diameter of the section was be- 

 tween one and two inches." 



This arrangement appears to have been continued through- 

 out this series of experiments ; for after discussing his results, 

 he says, "Inasmuch as the cusp-ray, within the crystal, corre- 

 sponds to a cone of rays without, it is evident that there must 

 be a converging cone incident on the first surface, equal to 

 that which diverges from the second." 



We see in the preceding discussions, that Professor Lloyd, 

 by experimenting with the view of verifying theoretical conse- 

 quences, has fallen into two eiTors; the one an error of assump- 

 tion, that a conical incident pencil of common light could be 

 refracted as one ray within the crystal ; and the other an error 

 of omission, in not discovering that, however small the incident 

 pencil may be, the refracted light consists of a larger cone and 

 a smaller one, nearly a cylinder. The first error arose from ap- 

 plying the reasoning of geometrical optics, for uncrystallized 

 media and common light, to the case of a double refracting me- 

 dium and polarized light; for Professor Lloyd himself had dis- 

 covered a peculiar state of polarization in the emergent pencil 

 (he says, " Thus it appeared that all the rays of the cone are 

 polarized in different planes"), and to have given the least 

 chance of success the incident pencil ought to have been in 

 the same state. 



To investigate the form of the refracted light in a more 

 severe manner, I placed an aperture made in a plate of metal, 

 with a small pin, about one-fiftieth of an inch diameter, at 

 seven inches from the flame of a lamp; and at sixteen inches 

 on the other side of the aperture I placed the crystal with 

 tinfoil attached to its first surface, and examining the light 

 which passed through a minute hole, about one-thousandth of 

 an inch in diameter, in the direction for refraction symmetrical 



