NA TURE 



May 



3. 1900 



the methods of mathematical analysis. So long as our 

 English university colleges are, to a great extent, in the 

 hands of oligarchies, who attach more importance to such 

 trifles as the handwriting and spelling of matriculation 

 or medical preliminary students than to higher scientific 

 study, such courses of training will only be accessible to 

 those who seek them in countries more enlightened in 

 the matter of scientific education than Great Britain. 

 We can readily imagine that Bjerknes' theories may find 

 their way into many transatlantic universities among the 

 "classics of science." They have, indeed, no small claim 

 to be regarded as classical. It is true, as Prof. V. 

 Bjerknes points out, that his father's and Kirchhoff s work 

 in several cases somewhat overlapped, but it would ap- 

 pear that in developing the theory of motion of spheres 

 in liquids as a basis for explaining the properties of 

 matter, Bjerknes stood entirely on ground of his own 

 making. Other theories involving the conception of a 

 continuous medium have sprung up ; we have had the 

 vortex-atom theory before us, and we now find it neces- 

 sary to postulate the existence of an ether, whose attri- 

 butes resemble those of an elastic solid rather than a 

 fluid. At the present time few will regard the hypothesis 

 of pulsating spheres as of more than classical interest. 

 As having been first developed in the face of a prevalent 

 belief in the doctrine of action at a distance, and as in- 

 genious methods of replacing this action at a distance by 

 the action of an intervening medium, the application of the 

 term " classical " to these investigations of C. A. Bjerknes 

 may not be altogether without justification. 



G. H. Bryan. 



PHO rO-MICROGRAPHY. 

 Photo-micrography. By Dr. Edmund J. Spitta. Pp. 

 xi -H 163. (London : The Scientific Press, Ltd., 1899.) 



A QUARTER of a century has now elapsed since 

 the renaissance of the art and science of photo- 

 micrography. Up to that time much of the best work in 

 this direction was accomplished in America by Lieut.- 

 Colonel Woodward, of Washington, whose successful 

 photographs of diatoms excited the admiration of all 

 microscopists who saw in his productions the faithful 

 delineations of those "markings" on them, on which 

 many hours of microscopical manipulation had been 

 spent in bringing their delicate tracery to a correct 

 definition. From that time to the present the fascination 

 of transferring the minutest details of histological and 

 biological science to the photographic plate has found 

 many ardent votaries, with the result of improved 

 apparatus and lenses corrected to such a degree of 

 accuracy for this work that sharp and well-defined 

 images can now be obtained in a manner that would 

 have been a boon and a revelation to workers twenty- 

 five years ago. 



Amongst the latest exponents of this branch of micro- 

 scopical science we must name that of the author of the 

 book under consideration. 



Dr. Spitta in this work on photo-micrography has 



dealt with the subject very fully and from a scientific 



standpoint, so that the student who takes up this branch 



of the photographic art is thoroughly furnished with all 



NO. 1592. VOL. 62] 



the information necessary to the accomplishment of 

 perfect work, leaving, however, only that amount of 

 personal experience to be obtained and which will be 

 demanded of every one who first embarks on this art, 

 and without which he is liable to be landed in many 

 difificulties. 



In Chapter i. the author deals with illuminants, a 

 by no means unimportant point for consideration ; for 

 although we have several good illuminants for low power 

 work, it is when we come to work with the highest power 

 objectives that either the lime-light or that of the electric 

 arc lamp must be employed to produce the best possible 

 results. These lights are not always readily accessible ; 

 but as the aspiring student most probably will try his 

 'prentice hand on low power work, the single wick lamp 

 burning the best paraffin oil will furnish him with a light 

 sufficiently rich in actinic rays that, provided the proper 

 length of exposure be given, will result in a very suc- 

 cessful negative. Dr. Spitta in Chapter ii. proceeds to 

 give directions for obtaining photo-micrographs by low 

 power objectives, dealing with this in such a lucid 

 manner that the student who closely follows his clear 

 description cannot fail in being rewarded by satisfactory 

 results, being assisted in his work by algebraical 

 formuke and illustrations of simple but effective 

 apparatus. 



Chapter iii. deals with medium power photo-micro- 

 graphy, and contains some very necessary cautions 

 relative to the avoidance of vibrations in the apparatus, 

 for, as the author observes, "when photographing at 

 1000 diameters, i/iooo of an inch shake in the specimeni 

 makes a shift of one inch in the photographic plate," or 

 he might have said in the photographic image; therefore 

 the absolute necessity of the most perfect stability, not 

 only in the apparatus but even in the studio, can be 

 readily understood and provided for — even a heavy tread 

 on the floor of an adjoining room being sufficient to dis- 

 turb the steadiness of the optical arrangement. Dr. 

 Spitta describes different methods whereby this difficulty 

 may be overcome. Allowance must also be made for the 

 expansion of the metal of the microscope from the heat 

 of the illuminant, for even in low power work, say of 250 

 diameters, the heat from the oil lamp must not be con- 

 sidered a negligible quantity, and must be considered so- 

 far that no photographic exposure should be attempted 

 till the metal has had time to become fully expanded. 



Chapter iv. is overloaded with woodcuts of different 

 makes of microscopes valuable as affording the studerit 

 a choice of various instruments, but by no means neces- 

 sary to his work, as any one of these is sufficient for 

 attaining good medium power work. This chapter alsa 

 deals with the subject of lenses and eyepieces and the 

 accessory fittings of the microscope generally ; but there 

 is one point that must have the greatest attention, and 

 that is the fine adjustment, and Dr. Spitta does well ia 

 laying great stress upon its importance ; nothing is more 

 embarrassing to the operator, when perhaps everything 

 else in the apparatus is working well, to find that the 

 fine adjustment by which he hopes to obtain that sharp 

 definition without which his work is valueless, is alto- 

 gether useless from faulty construction, and Dr. Spitta. 

 describes the various forms of this all-important additioa 

 to the photo-micrographic installation. 



