ILLUMINATION FOR THE MICROSCOPE. 



157 



and, second, a means of properly connecting it with the street 

 service. 



Having decided on the character and form of lamp desired, I 

 applied to the Edison Lamp Works to have it constructed accord- 

 ing to the plans and specifications which I furnished. These 

 were carried out as requested, and the result was a lamp of fifteen- 

 candle power, requiring a current of about three amperes under 

 a pressure of fifteen volts. 



The lamp in question possessed certain pecu- 

 liarities of construction, as will be seen by an 

 examination of the cut (Fig. 24). 



The glass bulb, instead of possessing the pear- 

 shaped form usually met with, is cylindrical, and 

 about three inches in length by an inch in diam- 

 eter. At first glance the carbon filament would 

 appear to have the ordinary horse-shoe form, and 

 to be of the usual length (four to five inches). 

 A closer inspection, however, shows that the car- 

 bon is actually but three quarters of an inch in 

 length, while the rest of the apparent filament is 

 composed of copper wire, arranged so as to hold 

 and support the carbon in a vertical position. It 

 will also be noticed that the carbon is much 

 broader and thicker than in the ordinary domes- 

 tic electric lamp. When this carbon is rendered 



Fig. 24. incandescent by the passage of a suitable electric 



The author's electric ^ -n , u <.i 1 • • 



illuminator. current, we will have, when the lamp is in posi- 

 tion, a vertical streak of light, of intense brilli- 

 ance, about three quarters of an inch long, and apparendy an 

 eigth of an inch wide. The minified image of this is focussed by 

 mirror or condenser on the object we desire to examine, and con- 

 stitutes "critical" illumination. If now we proceed to the 

 examination of the object, with, for instance, a quarter of an inch 

 objective, we observe that the field is not evenly illuminated, but, 

 instead, a central brilliant streak, on each side of which the light 

 is comparatively feeble. The portion of the object within the 

 area is now illuminated in the manner most favourable for the 

 revelation of its intimate structure. In systematic work, critical 



