Illuminating Apparatus. By J. W. Gordon. 427 



work under other conditions than those of precise focusing on the 

 stage of the instrument ; and the very considerable advantage of 

 being able independently to control the brightness of the illumina- 

 tion and the angle at which the light shall be incident upon the 

 object, is lost. This is, in fact, a more serious difficulty than is 

 commonly supposed in the way of high-power microscopic work ; 

 and what has here been stated in reference to the lamp-flame 

 applies, of course, with added force to such sources of light as 

 electric lamp filaments or Welsbach mantles. The diffraction 

 which they produce when thrown slightly out of focus makes them 

 wholly useless under those conditions of working, and the con- 

 traction of the illuminated field when they are in focus makes them 

 entirely unsuitable for the purposes of critical illumination. 



This difficulty has in practice been met by placing ground-glass 

 between the source of light and the condenser. So long as the 

 ground-glass remains out of focus it forms a most excellent light 

 source. But if it is brought into the position in which it yields 



Ground Glass 





Fig. 120. 



the brightest field its grain becomes conspicuous, and of course 

 destroys the image. For this reason a flame or filament covered 

 with ground-glass, in the usual way of employing that medium, has 

 only a very limited application in microscopy. 



The two great difficulties, then, against which provision has to 

 be made in devising a source of light for the Microscope are 

 (1) diffraction due to the post-ocular image of the filament when a 

 glowing filament is used ; and (2) the loss of light and intrusion of 

 the grain when a diaphanous screen is employed to diffuse the light 

 from the primary light source. 



Both these difficulties are met by the use of the speculum ex- 

 hibited this evening. The apparatus consists, as shown in fig. 120, 

 of a glass rod, one end of which is cut to a plane surface and finely 

 ground. Such a surface can be very strongly illuminated, for it 

 will bear exposure to very intense radiant heat. The small size 

 and symmetrical form of the exposed surface render it singularly 

 tolerant of this particular kind of hard usage, and the very con- 

 siderable conducting power of the glass rod prevents it from being 

 easily heated to fusing-point. It may thus be placed with perfect 

 safety within \ in. of a Xernst filament, and in that way it can be 

 made to receive a very intense illumination. In the illuminating 

 apparatus now under description this ground-glass surface becomes 



