32 ILLUMINATION. 



fice a slight rim projects into the interior, on which 

 brass discs having different sized central apertures 

 may be fitted. 



Another tube E carrying a disc of blue glass may 

 be made to slide within the tube 2. If the tint of 

 this glass is properly chosen it obviates the neces- 

 sity of using a part of the spectrum to produce 

 monochromatic light. 



The intensity of the colour is increased or dimin- 

 ished by increasing or diminishiug the distance be- 

 tween the blue glass and the aperture of the slot or 

 back of the condenser. 



The advantage of this sub-stage arrangement 

 is that no light can reach the object or enter the ob- 

 jective except through the diaphragm and condenser 

 themselves ; the ease and simplicity with which the 

 several adjustments are made, and the possibility 

 of adapting it to any existing microscope stand. 



Illumination. 



The best light for Micro-photography is sunlight, 

 but the difficulties of its employment in our climate, 

 more than counterbalance the excellence of its re- 

 sults. 



We shall give a description of the apparatus neces- 

 sary for the benefit of those who can afford sufficient 

 time during daylight for Micro-photography, or who 

 may be more favourably situated atmospherically. 



It should be borne in mind that the actinic power 



