February 6, 1908J 



NA TURE 



315 



Not so Mr. Conrady, who contributes to the present 

 volume a couple of chapters on the undulatory theory 

 of light, and on Abbe's diffraction theory of the micro- 

 scope image. Mr. Conrady " has no use " for the new- 

 theory, propounded long since by Dr. Altmann, and 

 only of value in that it called forth a complete and 

 overwhelming reply from .Abbe in his well-known 

 paper " Uber die Grenzen der geometrischen Optik." 

 He urges strongh- the adequacy of the diffraction 

 theory to explain all the observed phenomena, and is 

 emphatic as to the inapplicability to the microscope of 

 the theory of the .\irv diffusion-disc. 



This, however, is not the place to enter on this much- 

 discussed but fascinating topic, with which, indeed, 

 Mr. Spitta's book, from its plan and object, is but 

 little concerned. It is the practical rather than the 

 theoretical to which attention is directed, to the intel- 

 ligent handling of one of the most finished and deli- 

 cate of optical instruments. 



From this point of view, let us hasten to urge every 

 student of the microscope who wishes to gain a 

 thorough understanding of its principles and possi- 

 bilities and its defects, and every user of the instru- 

 ment who desires a work of reference to which he 

 may turn for an explanation of some unexplained 

 optical phenomenon, or for particulars of up-to-date 

 apparatus, to procure a copy of Mr. Spitta's book 

 without delay. It is a leisurelv book — an un- 

 friendly critic might even call it diffuse — but there is 

 scarcely a chapter which will not repa)' careful read- 

 ing; and when one comes to the chapter on " Test- 

 ing Objectives," one can but feel grateful to Mr. 

 Spitta for his admirable treatment of a difficult subject. 



-Mr. Spitta has called his work " Microscopy "; but 

 it is only of one branch of microscopy that he treats. 

 His subject is the theory and use of the microscope as 

 an optical instrument; with the preparation of objects 

 for the microscope he does not deal. The book will 

 be of much interest and of great value to many who 

 are in no sense " microscopists," but who use the 

 microscope as an accessory in other physical investi- 

 gation. The non-mathematician who desires to know 

 the meaning of the terms " numerical aperture," the 

 " sine-law," " resolving power," or to make himself 

 familiar with the essentials of the Abbe theory, will 

 find Mr. Spitta a satisfactory guide; and the micro- 

 scopist proper will find innumerable useful suggestions 

 as to the manipulation of his instrument. 



It will be well to indicate shortly the ground Mr. 

 Spitta covers. After a preliminary account of the ele- 

 ments of geometrical optics and the theory of the 

 simple microscope, he proceeds to deal with the com- 

 j)ound microscope in its modern form, fine adjust- 

 ments, mechanical stage, substage, objectives — 

 achromatic, semi-apochromatic, and apochromatic ; 

 dry and immersion — with details and illustrations of 

 the work of the best makers. In connection with ob- 

 jectives the chief optical properties, spherical and chro- 

 matic aberration, the sine-law, &c., are discussed. 

 Then follow chapters on numerical aperture, eye- 

 pieces, magnification — in which may be found the main 

 principles of the .\bbe theory — the substage condenser, 

 and methods of illumination — critical light, mono- 



No. iqq;, vol. 77] 



chromatic light, dark ground illumination, Rhein- 

 berg's multiple colour illumination, oblique light, il- 

 lumination of opaque objects, polarised light. Then 

 we come to " the use of the microscope," with which 

 mav be mentioned the valuable hints to workers with 

 which the volume concludes. The binocular micro- 

 scope and measurements with the microscope are 

 treated, and a long chapter is devoted to the discus- 

 sion and illustration of microscopes by different makers 

 for various purposes, which is a feature of the book. 



Then follows the excellent account of the testing of 

 objectives already referred to. Mr. Spitta confines 

 himself to the use of the .\bbe test-plate, and of speci- 

 fied test objects, but within these limits he goes into 

 the matter in detail and with admirable clearness, and 

 this chapter alone is sufficient to justify the work. The 

 section is illustrated in sixteen plates by a beautiful 

 series of photomicrographs. Mr. Conrady's two 

 chapters follow, with another on accessory apparatus. 

 The usefulness of the book is completed by a satis- 

 factory index. 



We have said enough to commend Mr. Spitta's 

 volume. It teems with " tips,", and is likely to com- 

 mand an even wider popularity than his previous 

 books on allied subjects. 



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(i) "T^HIS book is a collection of 3500 examples in ele- 

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