September 30, 1920] 



NATURE 



141 



be clearly seen. A synoptic table at the end of the 

 introduction shows where each section of the two 

 old editions is to be found in the new one, and 

 also what is now printed for the first time. 



As already mentioned, the first part is the 

 treatise of 1653 on refraction and telescopes. Of 

 this, Book 1. deals with refraction due to plane and 

 -[)herical surfaces and lenses. Huygens describes 

 \arious methods of determining the refractive index 

 of liquids or glass, but thinks it unnecessary to at- 

 tain great accuracj' in this, as the value of the index 

 varies slightly for different liquids and different 

 sorts of glass. He then investigates the two prin- 

 cipal problems of this book : the determination of 

 the foci of lenses, and of the places where the 

 images of objects in known positions are formed. 

 If this book had been published in 1653, Huygens 

 would have had undoubted priority for most of 

 his results, though he had (without knowing it) 

 been anticipated by Cavalieri as regards foci. But 

 the important propositions en the places of images 

 were quite new. In September, 1669, when 

 Huygens sent to the Royal Society a series of 

 anagrams to secure priority, Barrow's " Lec- 

 tiones Opticae" were in the press. In this 

 work there is a determination of the foci of a 

 spherical lens, and of the position of the image of 

 a luminous point situated on the axis of the lens. 

 And when the " Dioptrica " of Huygens was at last 

 iiiblished, in 1703, the "Dioptrica Nova" of 

 Molyneux (1692) and a paper by Halley in the 

 Phil. Trans., 1693, had also much diminished the 

 importance of Huygens's discoveries. 



Book II. is "On the Apparent Size of Objects 

 seen by Refraction." Among the contents is an 

 elegant theorem of far-reaching applicability. 

 Whelh an object is seen through a number of 

 lenses, and the positions of the eye and the object 

 are interchanged, while the lenses are undisturbed, 

 the apparent size of the object will be unaltered, 

 and the image will have the same position, upright 

 or inverted. A striking application of this was 

 made by Huygens in Book 111., where he shows 

 that the magnifying power of a telescope is the 

 r.itiM C.I the diameter of the object-glass to the 

 diameter of the pencil of parallel rays emerging 

 from the eye-lens. 



Book III. deals with telescopes and contains, 

 among other things, a description of the Huy- 

 ),'enian eye-piece. 



As descril)ed above, part ii. of Huygens's works 

 treats of spherical aberration, and part iii. of tele- 

 scopes and microscopes, representing the outcome 

 of his lifelong studies and experiments, made in 

 order to improve the theory and construction of 

 refracting telescopes. 



NO. 2657, VOL. 106] 



These two beautiful volumes reflect as much 

 credit as did the previous ones on both the editors 

 and the printers. The lengthy introduction and the 

 footnotes are most valuable contributions to the 

 history of optics. The numerous figures in the 

 text are facsimile reproductions of Huygens's own 

 rough diagrams. 



J. L. E. D. 



Analysis of Foods. 



Food Inspection and Analysis: For the Use of 

 Public Analysts. Health Officers, Sanitary 

 Chemists, and Food Economists. By Albert E. 

 Leach. Revised and enlarged by Dr. Andrew L. 

 Winton. Fourth edition. Pp. xix-H 1090 + xli 

 plates. (New York : John Wiley and Sons, 

 Inc.; London: Chapman and Hall, Ltd., 1920.) 

 Price 45*. net. 



IT is now some sixteen years since this work was 

 first issued, and in the interval it has become 

 favourably known in this country to analysts and 

 others concerned with securing the purity of food- 

 stuffs. In its general plan the book remains much 

 as it was when the first edition was reviewed in 

 these columns (Nature, vol. Ixxi., p. 50, Novem- 

 ber 17, 1904), though naturally there have been 

 many additions and amendments. The author and 

 reviser have brought between two covers some 

 essential information upon almost every subject 

 with which, in the exercise of his profession, the 

 food analyst is likely to be concerned — from the 

 "cutting up " of beef and mutton to the equip- 

 ment of a laboratory, and from the taking of a 

 photomicrograph to the use of a hydrogen elec- 

 trode. The more he employs the book, the more 

 the reader will be confirmed in the impression that 

 the literature of the subject has been well searched 

 and judiciously abstracted for him. 



No doubt this quality of comprehensiveness 

 largely accounts for the success which the work 

 has achieved. Still, the quality has its in- 

 herent defect. So many things are dealt with 

 that, even in this bulky volume of more than 

 a thousand pages, little space is left for dis- 

 cussion of the difficulties which arise either in 

 making the various analyses or in correctly in- 

 terpreting the results. It is here that the per- 

 sonal judgment and experience of the in- 

 dividual analyst must come in. The book gives 

 him valuable help, but of a generalised kind. It 

 puts up analytical signposts to indicate the high 

 roads for him, but when off the beaten track he 

 must find his own by-ways. 



In preparing the present edition Dr. Winton 

 has had expert assistance for the revision of various 



