22 ON THE ACTION OF TRANSPARENT BODIES 



peated trials, and as it would have been impossible to replace 

 them from the same source, the prosecution of the experi- 

 ments with fresh oils, sold under the same name, might have 

 introduced a new degree of uncertainty among the results. 



For all the purposes of the Practical Optician, the relative 

 position of the substances, where it has been determined by 

 experiment, is sufficiently accurate. Numerous combinations 

 for correcting the secondary spectrum, may be formed, ei- 

 ther by taking two media that have nearly the same ac- 

 tion upon green light, while they differ in their powers 

 of refraction and dispersion, or by adopting the ingenious 

 method discovered by Dr Blair, and fully described in the 

 Transactions of this Society. In the production of perfect 

 achromatic instruments, the optician cannot expect much more 

 aid from the principles of optics. The great, and almost the 

 only desideratum, is to obtain two kinds of glass, which shall 

 have the same action upon green light, while they differ suffi- 

 ciently in refractive and dispersive power ; and it is chiefly 

 from the labours of the Chemist, guided by the preceding in- 

 quiries, that such a discovery can be expected. 



There is still, however, another source o^ error, of which 

 neither the Theoretical nor the Practical Optician has hitherto 

 been aware. It arises from a crystallisation in the glass, which 

 is always accompanied with double refraction, and with a va- 

 riation of density. This crystallisation, which most frequent- 

 ly affects the flint-glass, and which can easily be detected by its 

 action upon polarised light, should be carefully removed from 

 every piece of glass used in the construction of optical instru- 

 ments, by annealing it in an oven of a high temperature, 

 where the heat is regularly and very slowly reduced. The ex- 

 periments upon which this opinion is founded, will form the 

 subject of another paper. 



TABLE 



