744 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The fifth chapter deals with numerical aperture and depth of focus 

 and gives all information required for ascertaining these important 

 properties of lens systems. 



Chapter YI., on Eyepieces, is remarkable for the clear way in which 

 the chromatic correction of oculars is described and illustrated, thus 

 throwing much-needed light on a subject which is much neglected and 

 frequently misunderstood. 



Magnification and its limits having next been dealt with, the author 

 proceeds in Chapter VIII. to describe and discuss the substage con- 

 denser, and to establish rules for its proper use, the subject being 

 followed up in the succeeding chapter by a description of auxiliary 

 lighting apparatus, such as bullseyes, lamps, heliostats, the author's 

 arrangement for obtaining monochromatic light by a direct-vision 

 Thorpe grating, etc. The question of the proper use of oblique light 

 and of dark-ground illumination is also dealt with, and the chapter 

 closes with a description of polarised light and its use in the Microscope. 



Chapter X. describes in great detail the proper manipulation in 

 setting up the instrument and preparing it for practical use, and is, as a 

 matter of course, full of valuable hints. 



Binocular Microscopes, in the English as well as in the Continental 

 form, are described in Chapter XI., whilst Chapter XII. deals with the 

 measurement of microscopical objects, the instruments used for the 

 purpose, and the units in which the measurements are usually expressed. 



Chapter XIII. contains illustrations and descriptions of many forms 

 of Microscopes, from the simplest to the most elaborate, classified 

 according to the purpose for which they are most useful. This chapter 

 concludes with statements of a large number of well-known micro- 

 scopists as to the kind of optical outfit which they find most useful for 

 their respective branches of research, statements which should be most 

 valuable to new beginners in these fields. 



The fourteenth chapter is one of the best in the book, dealing very 

 thoroughly with the testing of objectives, chiefly by means of the Abbe 

 test-plate, which is so little known in this country, and yet so valuable. 

 The various methods of effecting the chromatic correction are gone into, 

 and minute instructions given as to the methods of determining the 

 state of objectives in this respect as well as with regard to spherical 

 correction. 



Chapters XV. and XVI. are contributed by Mr. A. E. Conrady, who 

 here gives a short account of the undulatory theory of hght, and of the 

 principal results obtained by applying it to the theory of the Microscope, 

 which latter is treated from an essentially historical standpoint. 



A number of accessories of a specialised type, such as metal-holders, 

 spectroscopic attachments, and some of the latest novelties, are described 

 in Chapter XVII. 



" Hints " upon common faults and means of cure are dealt with in 

 the last chapter. It is needless to say that some of these hints are very 

 valuable. 



Seventeen magnificent photomicrographic plates illustrating the 

 principal test-objects call for special mention, and the very complete 

 index must prove a welcome addition to the book. 



