NEGATIVE MAKING 51 



nian form, and an Abbe chromatic sub-stage 

 condenser. The object chosen is a thin, trans- 

 verse section of an echinus spine Echinome- 

 tra lucuntur long esteemed by microscopists 

 as a very perfect test for flatness of field in an 

 objective in addition to its beauty. The mi- 

 croscope having been inclined to a horizontal 

 position, with the slide placed on its stage 

 and secured by the clips, we turn the mirror 

 aside in order to leave the under surface of 

 the stage quite free and unobstructed. The 

 radiant in this case a Welsbach burner, so 

 mounted as to bring its centre in a line with 

 the optical axis of the microscope is placed 

 about six inches from the stage. Upon look- 

 ing through the eyepiece and bringing the 

 object into focus, we find the illumination to 

 be somewhat uneven, and not altogether satis- 

 factory, caused by the presence of the Abbe 

 condenser, which is not well adapted to the 

 low power (f inch) we are attempting to use. 

 Removing this from its setting and closing 

 the stage iris diaphragm somewhat, the illumi- 

 nation will be found quite evenly distributed 

 over the whole field soft, yet brilliant. But 

 we also discover that only a fraction of the 

 object is comprised within the field of view. 

 Manifestly it is impossible to photograph it 



