ZOOLOGY AND BOTANY, I\IICROSCOPY, ETC. 505 



MICROSCOPY. 

 A. Instruments, Accessories, etc.* 



f3) lHuminating' and other Apparatus. 



Microscope Accessory.f — J. W. Evans, in a paper on the use of a 

 slit in determining refractive indices with the microscope, says certain 

 ■optical properties of crystals, and more particularly the refractive index, 

 may be determined either in the directions-image, often referred to as 

 the "image in convergent light," or in the ordinary object-image in 

 which the object itself is seen. In the former case, in which the index of 

 refraction is usually determined by means of the critical angle of total- 

 reflection, every point in the image corresponds to a single direction of 

 propagation of the wave-front through the crystal-structure and to the two 

 ■corresponding directions of vibration. One of these can, however, be 

 ■eliminated by the insertion of a nicol in an approximate position, and 

 thus all ambiguity in the determination of the refractive index is removed. 



In the object-image, however, the wave-fronts of the light by which 

 any point is illuminated have traversed the crystal-structure in an 

 infinite number of different directions, and to each of these correspond 

 two directions of vibration, so that there is no definite refractive index 

 which can be determined, and only approximate results can be obtained 

 whatever the method employed. 



This difficulty may be overcome to a considerable extent by placing 

 a diaphragm with a minute aperture in a position on the microscope 

 axis where a directions-image is formed, or in a focus conjugate to such 

 a position, so that the image of the aperture is seen in focus simul- 

 taneously with the directions-image, thus allowing only a small well- 

 defined portion of the latter to be visible or to be illuminated. 



By moving the diaphragm laterally, that is to say, at right angles to 

 the axis of the microscope, any point of the directions-image may be 

 rendered visible or illuminated to the exclusion of the rest, so that the 

 light which passes is limited to that which has traversed the crystal - 

 structure in what is practically one direction only, and with the help of 

 a nicol it may be restricted to that vibrating in one only of the two 

 corresponding directions of vibration. Then by an alteration of the 

 optical system between the diaphragm and the observer, an object-image 

 may be formed and observed, which is illuminated only by this special 

 light. 



The simplest method of applying these principles is to insert the 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



t Mineralogical Mag., xviii. (1917) pp. 130-2. 



