1821.] Crystallized Bodies on Homogeneout Light, \^^ 



mula (a) is the same for all the coloured rays, being zero at the 



commencement of the scale ; and hence it follows, as a necessary 



consequence, that the axes of all the colours are united in one, 



and the virtual and actual poles coincide with each other and 



with the centre. Did any sensible separation of the axes exist, 



it must become perceptible by the ellipticity of the rings wheii 



examined with homogeneous light of that colour from which 



they are furthest asunder ; but with the greatest attention, in 



plates of considerable thickness, I have not been able to observe 



the sHghtest shifting of the axis, or deviation from the circular 



figure, in passing from a red to a violet illumination. Moreover^ 



it is evident from the preceding theory, that any difference whicli 



may exist in their position, if too small to be sensible to the eye, 



can produce only an imperceptible deviation of tints. In fact^ 



if we suppose a = o for any colour, we get, for the position of 



the virtual pole, 



... /i ~ I' ' ^ '■ ' 



sm. 9 = </ — - — . sm. da 



6 being the angular distance of the point of coincidence from the 

 single axis of that colour. It is^ consequently, insensible when 

 S fl is so. Now, the polarising force of the apophyllite being 

 very feeble, the diameters of the rings in any plate of moderate 

 thickness must so far exceed this very minute quantity that the 

 virtual poles, did any exist, must fall within the hmit of the cen- 

 tral blackness ; the Newtonian scale would still appear to comt 

 rnence from the centre, nor could any sensible deviation from it 

 arise from this cause. 



2. When the prismatric spectrum is passed over an apparatus 

 containing a plate of this mineral, no perceptible change in the 

 magnitudes oi \hQ xvci^^ for different colours takes place. Hence 

 it appears that the value of the function / for all the coloured 

 rays is nearly alike. By measures taken on a divided apparatus, 

 a slight difference is observed. Taking the mean refractive 

 index R at 1*5431 (by a very careful measure) and the dispersion 

 J: R at 0-017, the formula 



7 . sin. «2 t . . , . 



L z:z t . = - . Sin. fl . tan. 9 



n . COS. (^ n 



gave as follows : 



This table, though not given as exact, owing to imperfections 

 in the specimen examined, agrees with the succession of tints^ 

 whichy as far as the fourth order, were, as follows : 



