ZOOLOGY AND BOTANY, MIOROSOOPY, ETC. 575 



Too much light, supposing the manipulations are tolerably well 

 attended to, will be manifest by the appearance of a multitude of 

 diffraction lines, and these, as a rule, may be recognized by their 

 extending beyond the objects observed. Under very high amplifica- 

 tions, involving the use of powerful eye-pieces, we can, of course, 

 make use of a little more of the solar beam. 



Another method of sunlight illumination will be found useful at 

 times — the " reflex " illuminator with direct sunlight. In this case the 

 solar beam can be received through the closed window and reflected 

 from the plane mirror. " This illumination is only suitable for work 

 with wide apertures, and over the most minute objects, and the mount 

 must be free from surrounding objects of a coarse character, else, from 

 the extremely oblique character of the illumination, these stronger 

 and coarser objects will project their strong shadows across the field, 

 causing nothing but confusion and chaos. With the genuine form of 

 the Wenham ' reflex ' an epithelial scale would hardly be recognized 

 were there several in the field. The principal advantage in the use of 

 the ' reflex ' with sunlight is in arriving at a knowledge of surface 

 markings, and for this purpose it is indeed very valuable." " The 

 mirror may be substituted for the hand-lamp when working in the 

 evening, but the most favourable results are obtained with the light 

 direct. This reflex and sunlight illumination is especially desirable 

 when one wishes to trace out structure situated in one particular 

 plane, to the exclusion of that lying in adjacent planes. In the 

 general squabble to produce the so-called penetration, this very 

 important item has been lost sight of." 



Blue-tinted Lamp Chimneys, Light Moderators, &c.*— Dr. J. E. 

 Smith, referring to the attempts made to modify artificial illumina- 

 tion by the introduction of blue-tinted chimneys, white-ground 

 illuminators, &c., says that he has patiently tried the entire list and 

 rejects them all, from the fact that there is no real advantage secured 

 by their adoption, which cannot be obtained in a simpler way without 

 them. The neutral tint " light moderator," so-called, is a pleasant 

 thing enough for use with moderate amplifications ; yet there is 

 nothing seen with it that cannot be as well shown without it. 



The blue-tinted chimney cuts down seriously the intensity of the 

 lamp illumination to an extent which will defeat the resolution of any 

 severe test, while, on the contrary, any and all work with the lower 

 powers can be as well accomplished without its aid. 



Thompson's Polarizing Prism.f — Neither the polarizing prism of 

 Nicol nor that of Foucault can be regarded as perfect. The latter 

 especially has so small an angular aperture available, as to be very 

 inconvenient for any but narrow beams of parallel light. Prof. S. P. 

 Thompson has sought to improve upon the existing forms of polarizing 

 prism ; and his investigations into the cause of their defects have led 

 him to produce prisms having a considerably wider effective angular 

 aperture. 



* 'How to See with the Microscope,' 1880, pp. 189-90. 



t Lond., Edin., and Dubl. Phil. Mag., xii. (1881) pp. 349-51. 



