1821.] Crystallized Bodies on Homogeneous Light, 121 



The substances, which I have examined most attentively, are 

 sulphate of baryta, nitre, mica, and Rochelle salt, and the sub- 

 joined tables of tints developed for different incHnations in plates 

 of the first and last of these, may serve as examples of the mode 

 of action of the respective classes to which they belong on hght, 

 and will afford data for some calculations to follow. The first 

 two columns contain the inclinations corresponding to siniilar 

 tints of the incident ray on the moveable plate which carries the 

 crystal, in the general apparatus imagined by M. Biot for obser- 

 vations of this kind. Were the plate cut in a direction precisely 

 perpendicular to the optic axis (or line bisecting the angle between 

 those of double refraction)^ and adjusted with perfect accuracy 

 on the instrument, the excesses or defects of these angles above 

 or below 90^ would represent the angles of incidence. Neither 

 of these conditions were, of course, exactly fulfilled. But it is 

 obvious that the small errors in these particulars (which were 

 ascertained not to excs^d 1° or 2°) must affect the computed 

 angles of refraction on^both sides of the perpendicular with, 

 equal and opposite errors. The same may be said of any error 

 arising from a slight prismasticity of the plate, which, however, 

 must have been extremely small, the plate having always been 

 rendered parallel by the delicate test of the sphserometer, within 

 a very few divisions.* Consequently, in calculating on these 

 data, the mean angle of refraction determined by the simultane- 

 ous use of both observations (their semi-difference being taken 

 for the angle of incidence), may be expected to differ from the 

 truth by an extremely minute quantity. The third column con- 

 tains the tint developed in the ordinary pencil, and the fourth in 

 the extraordinary. The last notices the remarkable points in 

 the system of rings to which the tints and angles in the other 

 columns correspond. The positions of the poles were determined 

 by interposing a red glass between the crystal examined and the 

 reflector used to polarise the incident fight. The glass used for 

 this purpose was of that kind occasionally found in old church 

 windows, and whose manufacture seems to be numbered among 

 arts now forgotten. It transmits almost the whole of the red 

 rays, and part of the orange ; while it completely stops all the 

 more refrangible colours. I have endeavouredin vain to procure 

 a specimen, whose fimits of transmission are more confined. 

 -Such are said to exist, though very rare ; and in the absence of 

 such, the indications of that employed may be taken to corres- 

 pond to the mean red rays. 



* Each equal to the 23809th part of an inch. * 



