APPARATUS FOR SCIENTIFIC DEMONSTRA TION 179 



the flame down to a small blue jet enclosed in a metal vase. 

 This also leaves the room in practically total darkness, with 

 no possibility of turning the light out, and with the power 

 of instantly turning it on full. It is advisable, however, to 

 keep this also constantly shaded from audience and screen, 

 during delicate experiments, that the sight of .the audience 

 may not be dulled. The effect which continual darkness (or 

 semi -darkness) has upon the ability of the eye to perceive faint 

 effects, is simply wonderful, and is not attended to in most 

 lecture theatres to the extent it should be. 



It is possible to arrange things in a permanent demonstra- 

 tion theatre or hall, so that the very act of switching off the 

 electric current, or turning off the gas from the lantern, shall 

 turn up the gas in either of the arrangements here described. 



CHAPTER XIII 



THE PROJECTION MICROSCOPE 



ABOUT the close of the year 1881, I was strongly urged by 

 several fellows of the Royal Microscopical Society and others, 

 who knew my proclivities, to turn my attention to the improve- 

 ment of the Projection Microscope, being assured that any 

 instrument which would display objects effectively even on the 

 scale of 600 or 700 diameters, would be an immense advance 

 upon anything then obtainable. By one or the other of those 

 interested, all the instruments then known were placed in my 

 hands, and the subject never having till then engaged my 

 attention, I was surprised to find how little had been done 

 in that direction. Not one of them, with the best lime-light 

 possible, would exhibit the bulk of those slides which any 

 demonstrator with a serious purpose in view would desire to 



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