•460 Analyses of Books. [June, 



bodies, and with an account of the experiments which were made 

 with it. Till witliin these few years the subject of dispersive 

 powers, perhaps the most curious and useful in physical optics, has 

 been investigated solely with the view of discovering achromatic 

 combinations for the improvement of the telescope. Ihe dispersive 

 powers of different kinds of glass, and of a few fluids, were nume- 

 rically ascertained ; but no attempt was made to investigate the 

 subject as an iinportant branch of physics. Dr. Wollaston had the 

 merit of beginning this interesting inquiry ; and he determined tlie 

 order of dispersive powers for 33 substances, without, however, 

 giving any numerical estimate of their magnitude. By means of 

 the instrument which has been mentioned, Dr, Brewster has ascer- 

 tained in numbers the dispersive powers of 13/ substances, the 

 greater part of which were never l)efore examined, and has obtained 

 many results of a most unexpected and singular kind. 



T!ie fourth chapter of the work contains a series of experiments 

 on the polarization of light by reflection and transmission, on the 

 depolarization of light by transparent bodies, and the colours exhi- 

 bited by mica and topaz when exposed to polarized light. This new 

 branch of optics owes its existence to M. Malus, and has been 

 diligently prosecuted since his death by the author of the present 

 work. In the yliinals of Ph'ilosujjhy, vol. iii. p. 6, we have 

 already had occasion to give an account of the leading results of 

 these experiments, and must therefore content ourselves with re- 

 ferring the reader to the numbers which we have now mentioned. 



The fifth and last book is occupied with an account of new 

 telescopes and microscopes. 



In the first chajitcr the author gives an account of a series of ex- 

 periments on the action of various bodies on the difitrently coloured 

 rays, made with a view to the improvement of the achromatic 

 telescope. Clairaut and Bcscovich had ascertained, by unquestion- 

 able experiments, that in every achromatic telescope formed of 

 crov.'n and flint glass a considerable quantity of colour remained 

 uncorrected, and that this uncorrected colour, or secondary sj)ec- 

 trum, as it has been called, arose from the same coloured spaces 

 having different magnitudes in equal spectra formed by the two 

 kinds of glass. In consequence of this difference of action on the 

 differently coloured rays, it is impossible to produce a telescope 

 perfectly achromatic by flint and crown glass, fcoscovich has shown 

 how the uncorrected colour may lie diminished, or how three of the 

 colours have been united, by using three substances having different 

 refractive and dispersive powers ; but this method has never been 

 carried into effect, and is of no practical use. Dr. Blair obtained 

 the same results with Boscovich, and has pointed out a very inge- 

 nious method of removing the uncorrected colour by a double com- 

 bination of fluid lenses j but this method is evidently too compli- 

 cated, and will probably never be attempted by any practical 

 optician. 



In prosecuting^ these experiments Dr. Brewster has examined the 



