366 E. M. NELSON ON A NEW OBJECT GLASS BY ZEISS. 



boides), so that this new kind of illumination must not be 

 confused with that from a condenser considerably decentred, 

 with the illuminant so placed that the light passes through the 

 condenser obliquely, a form of illumination old and well known, 

 or rather which used to be well known. 



Although there is no difficulty in executing the necessary 

 manipulation, the explanation of how the result is obtained is not 

 so easy. First, no direct light from the flame enters the field, but 

 it must be remembered that it is not a dark-ground image we are 

 dealing with ; for if it were high resolution would fail, as Mr. 

 W. B. Stokes has pointed out. The field is not dark, neither is it 

 light, but it is a sort of glow ; from whence does this glow come ? 

 At first it was thought that it must arise from internal reflections 

 in the front lens of the object glass, and that the lens was acting 



Fig. 2. Fig. 



3. 



as its own lieberkiihn, as in fig. 2. But further experiments have 

 proved that this is not the case ; no doubt some light may travel 

 in that manner, but the amount that does so is quite small, and 

 wholly insufficient for the purpose. The main body of this light 

 is present owing to spherical aberration in the condenser, which 

 gives rise to a very oblique beam, as in fig. 3. For this kind of 

 illumination therefore a condenser with spherical aberration is to 

 be preferred to one more aplanatic. 



There can be no question about extraneous light from the 

 illuminant having anything to do with it, for when a metal 

 screen, with a slit the size of the edge of the flame, was placed 

 close to the chimney, no difference in the effect was observed. 



This kind of illumination will be of service, for it will enable an 

 observer to obtain high resolution with a dry condenser, in an 

 instant, without the troublesome manipulations usually necessary. 



Journ. Quekett Microscopical Club, 8a: 2, Vol. XII., No. 7", November 1914. 



