382 Notices respecting New Books. 



Chapter iv., on " Dispersion," contains an account, fully illus- 

 trated, of some of the most used Spectroscopes, and an outline of 

 Spectroscopy — under the title " Study of the different sources of 

 Light " — mainly taken from a special work by M. Salet. Then 

 follows an account of various methods which have been employed 

 for the measurement of indices of refraction, with a discussion of 

 the results, as of the formula) which have been proposed for con- 

 necting the variations of the index with those of temperature and 

 density. A third section of the chapter is devoted to " Achroma- 

 tism" and instruments for measuring dispersive powers. Dollond's 

 achievement is by an oversight, or misprint, dated 1557 for 1757. 

 Here it may be remarked that M. Violle is usually full and exact 

 in his references to original memoirs and their dates, as becomes a 

 pupil of M. A^erdet ; but the careful reader will occasionally miss 

 those verifications of statements taken, presumably, from predeces- 

 sors ; as when he is told, " Ptolemee avait coustate la reciprocite des 

 rayons incident et refraete:" " d'aprcs Aristote, toute couleur ctait 

 melange de blanc et de noir. Ce tut aussi 1'opinion de Goethe" — 

 without any reference for the statements. Or when the authority is 

 quoted but without addition of page or edition; as : "Koger Bacon 

 le premier a donne la place exacte du foyer d'un miroir concave 

 dans son livre, dedie a Clement IV., Opusmajus, 1267." " Fermat, 



Littera3 Paris, 1667." " Klingenstierna K. Sv. V.-Ak. 



Handlingar, 1754;" and so on. Finally, Section IV. of this im- 

 portant chapter deals with Colours (the true theory of which is 

 assigned to Malebranche), and the methods which have been given 

 for calculating the tint of a given mixture. Here, and in the section 

 of the next chapter devoted to the Eye and Vision, reference is 

 freely made to the French translation of Von HelmhohVs work on 

 Physiological Optics, and to Clerk Maxwell's researches. In this, 

 as in several other parts of the treatise, it has been found necessary 

 to anticipate some of the results of the Wave theory. 



The concluding Chapter v. is occupied with M Optical Instru- 

 ments " other than those described in the preceding chapters ; 

 preliminary to which the Eye, with the optical means of meeting 

 its defects, and the Stereoscope are treated of in the first section. 

 In the second section the method of treatment is founded on the 

 consideration of the single equivalent lens. The "loupe" or 

 magnifying-lens, the eyepieces of Bamsden and Huyghens, 

 Microscopes with some account of the recent improvements, 

 Kefracting and Eeflecting Telescopes, are successively dealt with 

 in as much detail as a text-book on general Optics can afford. A 

 sketch of the history of Optical Instruments, down to the " Lunette 

 Coude'e " of M. Loewy, occupying some six pages, follows ; and a 

 sketch of apparatus for Projection and Photography closes the 

 volume. 



From this abstract of the contents it will appear that there 

 is no English book which covers the same extent of ground with 

 equal fulness of treatment and illustration. j j \vr 



