THE PROJECTION MICROSCOPE 199 



frequent turning, and the best plan is to give a fresh surface 

 for every slide. 



This all sounds rather formidable ; but practically the 

 demonstrator does it all without thinking about it, in a second 

 or two. It is done so easily that, although my experience 

 has been only that of rare and exceptional occasions, I have 

 found no difficulty in exhibiting fifty slides representing organs 

 of insects, with powers varying from 300 to 1,500 diameters, 

 during a lecture of an hour and a half, performing all 

 manipulations whilst explaining the slides, and with no 

 assistance whatever beyond that of a friend to hand me each 

 slide in order, wiped clean, in exchange for the one just used. 



Unless the operator's sight is unusually keen, an opera- 

 glass will be found of very great assistance in focussing the 

 image on the screen. 



It has already been said that the lowest sufficient power 

 should be preferred ; but a certain ' scale ' must be had to be 

 visible on the screen, and experience must decide what is this 

 necessary scale. It is not wise to attempt unknown slides 

 without rehearsal, after which the power required for each 

 should be marked on the label. Then a list should be made 

 out of every object in turn, with the power required for it; 

 and if the lecturer has to be his own demonstrator, it is 

 generally possible, by a little alteration in the arrangement of 

 the lecture, without any detriment to it, to bring most of the 

 objects to be shown under similar power more together than 

 at first, and so avoid too many sudden changes of objectives 

 for single slides. A series of demonstrations should always be 

 revised in this way, as an avoidance of needless changes not 

 only saves trouble, and wear and tear of apparatus, but enables 

 more time to be given in obtaining the very best effect, which 

 cannot be obtained with a fresh power in a moment of time. 



Often it is advisable to give a general view of the whole 

 slide with a low power, and afterwards to magnify much more 

 some portion of it. Sometimes it may suffice to add an 



