96 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the distance from the object 2*5 mm. The cone of rays, seen broad- 

 side, is shown in I. (fig. 18), and end-on in II., B being the base (i.e. 

 a in fig. 18). In III. is represented a cone of rays proceeding from a 

 base 2 ■ 5 mm. under similar conditions ; the vertical angle of the cone 



O 



T 



A 



B 



I 



c 



m 



Fig. 17. 



Fig. 18. 



is obviously much more obtuse. This latter form is to be recommended 

 for objectives of high aperture, and for objects visible by absorption 

 (e.g. coloured preparations), the narrow filament being better for objects 

 affected merely by refraction (e.g. diatoms, uncoloured preparations, &c). 



When the lamp is brought right 

 under the object, a drop of cedar- 

 oil will bring lamp and object- 

 holder into close connection, and 

 thereby much increase the effect 

 of the light. 



The lamp is supported on a 

 stand (fig. ID) by means of an 

 arm F, clamped by a screw E. 

 The heavy foot-plate is also a 

 resistance-block, and has a con- 

 tact-key K, thus allowing any 

 desired brightness of light to be 

 obtained. The arm F has a 

 lateral motion by means of the 

 spring C, whereby the position of 

 most advantageous oblique illu- 

 mination may be found. In Gr 

 the lamp may be rotated and 

 clamped for application with the 

 vertical illuminator. The holder 

 B is so fitted with contact-springs that the lamps have merely to be 

 inserted. The resistance-block is provided with a divided circle, so 

 that the degree of illumination can be always regulated. An accumu- 

 lator is recommended as a light-source. Among other advantages 

 possessed by the apparatus, such as cheapness, simplicity, and constancy, 



Fig. 19. 



