244 M, G. Wertheim on the double Re/faction 



the weight-chest reposes on its support, two images of the aper- 

 ture of the diaphragm placed behind the Nichol, a white or ordi- 

 nary image, and a black or extraordinary one, are seen. When 

 the cube is of glass, a gray tint is perceived on the latter 

 image ; but this effect, due to a residue of temper, may usually 

 be neglected in the case of well- annealed glasses, as we shall see 

 further on. 



As the pressure augments, the two images become successively 

 coloured with all the colours of thin plates ; the ordinaiy image 

 always taking the colours of transmitted light, and the extraor- 

 dinary image presenting the complementary tints, or the colours 

 of the^ings in reflected light. 



Tlie two images exhibit flat tints, if the experiment is well 

 made, when the pressure is exerted in a uniform manner, and 

 the substance is very homogeneous ; but if the least irregularity 

 exists, it is discovered by the experiment itself, and this, I think, 

 renders this method preferable to any purely mechanical means. 

 Thus, when the upper plane b deviates in a slight degree from the 

 horizontal line, and when consequently the charge is thrown more 

 upon one of the edges of the parallelopiped, we see at the same 

 side, in each image, a colour more elevated in the scale than on 

 the side opposite. This irregularity usually indicates that the 

 piece is not exactly in the middle of the press, at least that the 

 two horizontal faces of the piece possess some slight defect of 

 parallelism. 



When the piece, at the same time that it is compressed, takes 

 a transverse flexure, — which is often the ease, notwithstanding 

 their small height, — however small this flexure may be, coloured 

 bands make their appearance similar to those obtained by 

 means of the press of Fresnel : these bands sometimes vanish 

 by a slight displacement, or by the turning of the piece j but in 

 many cases it is necessary to cut the faces anew, or to reject the 

 piece altogether. Finally, diflerent colours are sometimes ob- 

 served in the two images which are quite irregularly distributed, 

 and which are a certain indication that some defect of homoge- 

 neity exists in the substance. 



Method employed to measure the artificial double Refraction. 



We have already said that the colours follow the scries of tints 

 in coloured rings. Newton having measured the thickness of 

 the plate of air which transmits and reflects each of these comple- 

 mentary colours, it is natural to attempt to make use of these 

 measurements to express numerically each tint obtained either 

 by compression or by traction. The following table contains 

 some of these determinations; the thickness of the plate of 

 air e is expressed in millionths of the English inch, and the 



