10 Transactions of the Society. 



length of the light used is of value. In terms of N. A. a reduction in 

 wave-length of the illuminant from 550 micro-millimetres to 450 

 micro-millimetres is equivalent to an increase of the aperture from 

 1"40 to 1"70 approximately. The most luminous part of the spectrum 

 is the region about 550 micro-millimetres wave4ength. In prac- 

 tice the use of screens or other devices which cut out this part of 

 the spectrum seriously reduces luminosity, so that it is necessary 

 when experimenting with colour-screens to have a variable illumi- 

 nant so that an increased emission of light may be secured to 

 compensate for the inevitable reduction in luminosity. I have 

 reason to believe that this factor, and the varying sensitiveness of 

 the eyes of different observers to different regions of the spectrum, 

 accounts for the variation in results obtained by the use of colour- 

 screens ; but in my own case I find such methods invaluable. I . 

 am afraid I have taken up too much time with generalities in con- 

 nexion with ultra-microscopy, but I feel sure that this branch of 

 work has a great future before it. It is usually regarded with a 

 certain amount of awe by the average worker, but I can assure 

 you such an attitude is not justified. It has difficulties certainly, 

 but they are easily mastered. The instrumental needs are not 

 great, and anyone with a taste in that direction could improvise an 

 efficient apparatus without difficulty. In another direction, the 

 use of ultra-violet light and other short radiations, the possibilities 

 are even greater. I have already brought this branch of work 

 before this Society on several occasions, so I will content myself 

 by showing you a few slides of later results. The work has been 

 almost stopped during the war, but I am in hopes of continuing 

 my experiments almost immediately. 



To most of us, however, are the limitations I have referred to, 

 the absolute limits that our microscope can attain, the ones we are 

 continually or even occasionally confronted with ? I think not. 

 Most of us are confronted with difficulties owing to our failure or 

 inability to take full advantage of the facilities that the microscope 

 offers to us. To obtain the utmost efficiency in ordinary every- 

 day practice the conditions with which we have to comply are 

 relatively simple and few. Firstly, we must have a solid cone of 

 illumination which is sufficiently large to utilize the utmost 

 possible working aperture of our objectives. Secondly, all the 

 optical systems employed, whether objective, eye-piece, or illumi- 

 nating system, must have their optic axes in exact alignment. 

 Tliis condition of affairs is known in this Society, but not so 

 widely outside it, as critical illumination. 



In this connexion it is interesting to note that any want of 

 •decentration, if it is only in a lateral direction — by that I mean 

 that the optic axes remain parallel, but are not co-incident — is not 

 so serious as a deflection of one optic axis, so that it is no longer 

 parallel but inclined to the axis of the microscope. 



