142 Amdyses of Books. [Aug. 



as much water, after distillation carried nearly to dryness, 

 and digestion with black oxide of manganese and re-distil- 

 lation, afforded an acid of the specific gravity 1*060. This 

 acid when neutralized with a solution of carbonate of potash, 

 and the addition of some pure colourless potash lye, acquires 

 no colour, and continued in this state for several weeks. 

 As the expense of the galls would be considerable, Prlickner 

 recommends as a substitute, a decoction of oak bark, which 

 he found to afford a precipitate, or a decoction of the fallen 

 catkins of the common alder (Alnus glutinosa.) 



Article VIII. 



ANALYSES OF BOOKS. 



I. — Optical Investigations. By G. H.S.Johnson, M.A., 

 Queens College, Oxford. Oxford: Talboy's, 1835, 

 p.p. 44. 



II. — Optical Investigations. Caustics. By the same. p.p. 33. 



The two tracts above named are remarkable, in several points of 

 view. In the first place, they are an encouraging specimen of the 

 increasing attention paid to physical science in the University of 

 Oxford, where, from a variety of untoward circumstances, (into 

 which we have not, at present, leisure to enter) it has never flourished 

 to any extent. In the second place, they are of such a character as 

 would any where claim for their author, a place in the highest rank 

 of mathematical talent. 



In this brief notice it is not our intention to enter upon a minute 

 analysis, or to pursue any critical details ; this could hardly be done 

 in an intelligible way, within a short compass, and we would rather 

 refer those of our readers who are interested in the subject, to the 

 original tracts. We will merely state their nature and object. 



He first examines the formulae which determine the direction of a 

 ray of hght after refraction, at a spherical surface. These are inves- 

 tigated in a very general form, and with express reference to the use 

 of the negative sign. The author also inquires into the approxima- 

 tion commonly used, by neglecting certain terms in the expressions, 

 and distinguishes particularly the value of the terms neglected, which 

 are shewn, in certain cases, to be so great as to involve serious errors. 



The second tract is, perhaps, the most strikingly elegant specimen 

 of analysis, and discloses a new method of investigating caustics, which 

 is of considerable generality, and remarkably simple in its application, 

 some instances of which are given. 



On the whole, we earnestly reccommend both tracts to the careful 

 study of those conversant with mathematical optics : they form an in- 

 dispensable supplement to all existing treatises. 



