ACCESSORY APPARATUS. 



119 



The light which enters at the lateral aperture (A, a) falling upon the 

 oblique surface of the disk (c, I), is reflected downwards, and is concen- 

 trated by the lenses of the Objective upon the object beneath. The 

 lateral aperture may be provided with a diaphragm, having a series of 

 apertures, for diminishing the false light to which this method is liable; 

 or a screen with a small aperture may be placed at any distance between 

 the lamp and the Illuminator, that is found to produce the best effects. 

 In using this illuminator, the lamp should be placed at a distance of 

 about 8 inches from the aperture; and when the proper adjustments 

 have been made, the image of the flame should be seen upon the object. 

 The illumination of the entire field, or the direction of the light more or 

 less to either side of it, can easily be managed by the interposition of a 

 small Condensing lens placed at about the distance of its own focus from 

 the lamp. The Objects viewed by this mode of illumination with dry- 

 front objectives, are best uncovered ; since, if they are covered with thin 

 glass, so large a proportion of the light sent down upon them is reflected 

 from the cover (especially when Objectives of large angle of aperture 



Fifl* 0L 



Beck's Vertical Illuminator. 



are employed) that very little is seen' of the objects beneath, unless their 

 reflective power is very high. With immersion objectives, however, 

 covered objects may be used; and the author has seen a more perfect 

 resolution of difficult tests by this mode of viewing them (first suggested 

 by Mr. Morehouse, of Wayland, New York) than by any other. 1 

 Another method of Vertical Illumination long since devised by Mr. 

 Tolles has recently been brought into notice by Prof. W. A. Eogers, of 

 Boston, U. S. It consists in the introduction of a small rectangular 

 prism, resembling that of Nachet's Binocular (A, Fig. 27), at a short dis- 

 tance behind the front combination of the Objective; so that parallel rays 

 entering its vertical end-surface, pass on between the parallel horizontal 

 surfaces, until they meet the inclined surface by which they are reflected 

 downwards. In passing through the front combination of the objective, 



1 " Journ. of Roy. Microsc. Soc.," Vol. ii. (1879), pp. 194, 266. 



