ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 723 



duced and clearly illustrated. Following this is a chapter upon image- 

 formation by means of lenses, or, as our author says with greater 

 exactitude, by means of " lens-armed " apertures. The essential im- 

 portance of the aperture in the formation of an image is never allowed 

 to escape the reader's attention, and leads in the end to a notable eluci- 

 dation of some of what are commonly supposed to be the more abstruse 

 problems in the theory of the Microscope. But it is true — and a truth 

 of which the reader of this book grows very conscious as he proceeds — 

 that abstruseness is a matter depending almost wholly upon the point of 

 view and mode of approach. Many things which seem remote and 

 abstract when treated merely as aberrations become very concrete and very 

 intelligible when presented as fundamental facts. The question whether 

 a particular phenomenon shall take rank as the rule or the exception is 

 usually a question simply of the point of view. Sir A. E. Wright 

 chooses to build up his theory of the Microscope about an aperture as 

 its essential element. The various lenses and combinations of lenses 

 which enter into the composition of the instrument are merely so many 

 appliances for supplying the deficiencies of the "vacant aperture." 

 They improve the definition of the image which the simple aperture 

 yields, and increase the resolving power of the beam which it transmits, 

 but the theory of the instrument and of its image is nowhere involved 

 with the theory of lenses ; and the reader — working out by easy steps 

 the properties of the simplest imaginable images — finds that he is 

 successfully grappling with some of the tougher problems of image 

 formation almost before he is aware that he is fairly launched upon 

 the subject. Thus the seventh chapter, which for the first time in- 

 troduces the lens and its function to the reader's notice, puts him in 

 possession, and almost without effort, of matters so " abstruse " as 

 Huyghen's law of wave-front propagation, and La Grange's numerical 

 relation between magnifying power and the diameter of the Ramsden 

 disk. 



Succeeding chapters treat of the defects which occur in the image 

 through aberration caused by the lens, and diffraction produced by 

 the aperture — in connection with which latter subject a singularly 

 complete series of experiments serves to illustrate the effect which 

 the form and dimensions of the antipoint have upon the appearance 

 of the image. In this connection the " Abbe theory " comes up for 

 discussion, and an exposition of the optical system of the eye, and 

 of the psychological factors involved in the use of the Microscope, 

 completes what may be called the abstract theory of image-formation 

 with the aid of lenses. What is really a subdivision of this second 

 part commences with the eleventh chapter, in which the simple Micro- 

 scope is discussed. Following upon this by a regular development comes 

 a chapter dealing with image-formation in the compound Microscope, 

 and another in which the anatomy of the compound instrument 

 is discussed in detail. We thus come to the fourteenth chapter, 

 which deals with the much-neglected subject of dark-field illumina- 

 tion, and following this is a chapter in which the corners of the 

 subject are swept up by the discussion of various adjustments neces- 

 sary for securing a critical image. The sixteenth (and last) chapter is 



