34"i Mr. Potter on Conical Refraction. 



of purchasing some crystals of arragonite, in order to have 

 pieces of larger dimensions than I possessed before, and which 

 from perusing Professor Lloyd's paper, in the seventeentli 

 volume of the Transactions of the Royal Irish Academy*, I 

 conceived to be necessary. The prepossession I had con- 

 ceived in favour of Fresnel's law, as expressed in the equation 

 to the wave surface in such crystals, was, however, changed 

 into distrust, when I was enabled to observe the phEenomena. 

 I employed a different mode of experimenting from Professor 

 Lloyd, on commencing ; and possessed more confidence in the 

 properties of an eye-lens. Such a lens exhibits to the sight 

 what takes place at the focus of the eye and the lens, as an 

 optical combination, when there is a real image; and when 

 there is a virtual image, it is exhibited such as would have 

 really arisen if the rays of light in falling upon the eye-lens 

 had proceeded in a similar manner from a real image. The 

 transition from a virtual to a real image, and vice versa, occa- 

 sions no difficulty ; for both arise from the form of the pencil 

 of rays, as it falls on the eye-lens ; which is, in fact, the thing 

 concerned in the representation of the image to the sense of 

 vision. 



Professor Lloyd passed a pencil of sunlight through the 

 crystal, and received the transmitted pencil on a screen of 

 ground glass, in most of his experiments, and only mentions 

 using a lens whilst seeking for the conical refraction within 

 the crystal which gives a cylindrical refraction in air: and 

 then soon gave it up again for sun-light. I have chiefly worked 

 with an eye-lens, or else for micrometer measures with a com- 

 pound microscope ; and having tried sun-light also, I con- 

 cluded that it bears in accuracy some such relation to the 

 former method as a camera-obscura bears to a telescojie, and 

 yields a less perfect result; but one which, as far as it goes, is ex- 

 actly the same as that from the other method. With the screen 

 you can only examine real images ; with the lens you can ex- 

 amine virtual ones also, and that in a more critical manner. 



Arragonite having been judiciously chosen by Professor 

 Lloyd, on account of the considerable angular separation of 

 its optic axes and its double refractive energy, as well as on 

 account of the refractive indices, in the principal directions of 

 the crystal, having been determined by Professor Rudberg, 

 I have continued to use it solely. I have employed slices of 

 the mineral cut perpendicularly to the axis of the crystal,which 

 is parallel to the line bisecting any pair of optic axes, and of 

 the thickness one-seventh, one half, and five-sixths of an inch. 



• [Prof. Lloyd also described these phaenomena in L. and E. Pliil. Mag., 

 vol. ii. pp. 1 12, 207.— EuiT.] 



