ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



251 



this took the form of a resistance coil, but it struck me that all the 

 current passing through the coil was wasted, so it was replaced by a 

 lamp, which could light the Microscope and also be the resistance for 

 the warm-stage. For the suggestion that the lamp should be of the 

 Nernst pattern, I am indebted to my brother, Professor Ronald Ross. 



" The lamp fills another purpose besides illuminating the Microscope 

 and regulating the amount of current to the warm-stage : it simplifies 

 the question of a change iof voltage. Suppose an instrument were 

 procured for a current of 100 volts, and one wished to use it with a 

 current of 230 volts, all that would be necessary would be to change 

 parts of the lamp, and the apparatus is ready for use. 



" The lamp is mounted on an oak base, and is supplied with two 

 switches, one for the lamp and one for the warm-stage.'" 



Fig. 49. 



Improved Methods of Working with the Vertical Illuminator.* 

 Method I. — With the image of a stop. Method II. — With the stop 

 and the vertical illuminator. 



The accessories necessary for Method I. are (1) source of light ; (2) 

 carrier for stop ; (3) condenser ; (1) vertical illuminator. The con- 

 denser is first set between the light and the vertical illuminator, so that 

 it forms an aerial image of the source of light at a distance from the 

 vertical illuminator equal to that from the vertical illuminator to the 

 top of the eye-piece. The carrier for the stop is then placed between 

 the light and the condenser in such a position that its aerial image 

 is exactly adjusted and falls sharply in focus at the back lens of the 

 objective. This will give an effect precisely the same as placing a 

 stop or diaphragm over the vertical illuminator itself, while the upward 

 path of the rays from the object to the eye is unimpeded. 



The accessories necessary for Method II. are (1) source of light ; 

 (2) bull's-eye condenser ; (3) vertical illuminator with stop or diaphragm 



* Knowledge, ii. (1905) p. 43. 



B 2 



