1020 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Eack-and-pinion movement was also found to be necessary, so that the rays 

 might be properly focused on either side of the object. The lenses used 

 should be large enough to take in the whole cone of the principal condenser, 

 and for the higher powers it is requisite to combine two or three of them 

 together. The highest as well as the lowest powers may thus be made 

 useful for lantern projections. Mr. Kingsley stated in his paper upon this 

 subject at the time I have just named that he could transmit as much light 

 through the higher as through any of the lower powers, and gave diagrams 

 of the arrangement which he made use of. 



So much for the past ; now we come to the present. The objectives 

 which I shall use this evening are 2 in., 1 in., and 4/10 in. The 2 in. 

 requires the substage lens to be a little over 2 in. focus, If- in. diameter, 

 plano-convex. A similar kind of lens. If in. focus, proves in my hands to 

 be a good all-round condenser for all powers from 1^ in. up to 4/10 in. 

 objectives. By liberal use of the rack-and-pinion and of the concave lens 

 to be presently described, this substage lens gives the most brilliant results 

 throughout this wide range of powers. The 1/4 in. objective, when it is 

 desirable to use it for photographic purposes, requires two lenses ; the back 

 one to be 2J in. focus and If in. diameter, and the front one 1^ in. focus 

 and 1 in. diameter, both plano-convex. This also makes a good condenser 

 for the 4/10 in. objective. All the lenses must have the curved surfaces 

 turned towards the lantern. The luminant goes to within If in. of the 

 back lens of the principal condenser with the 2 in., and to within 2 in. 

 with the other two objectives. I have tried it closer than this, by using a 

 back lens of shorter focus, without advantage — in fact, considerably other- 

 wise. If a flint concave lens is placed in the cone of rays about one or two 

 inches before the really active ones begin to cross, the light is much 

 improved. The concave which I use is about 6 in. focus and If in. 

 diameter. It is so placed in the tube which carries the other substage 

 lenses that its distance from the principal condenser can be altered so as to 

 modify the length of the cone of rays to adapt the focus of the other lenses 

 to the objective when they do not exactly meet its requirements. The 

 concave lens was, I believe, first introduced into the lantern cone of rays by 

 J. T. Taylor in 1866, for the purpose of parallelizing them, but I do not 

 use it for any such purpose in this lantern Microscope. In my lantern 

 polariscope I imitate Taylor in the use of the concave, but here the purpose 

 served is quite a different one. My lantern condenser is 3|- in. diameter, 

 with a plano-convex 3^ in. diameter and 7 in. focus, mounted upon the back 

 of the tube which carries the other lenses. 



In lantern Microscope projection three things are essential. The first 

 is brilliant illumination, the second large amplification, and the third clear 

 display of detail. But brilliant illumination does not mean a dazzling 

 display of light upon a large white screen, showing a dark, patchy outline 

 of an object, without detail. Objects shown in this way are far inferior to 

 an enlarged woodcut. The light must be made to enter the object so as to 

 bring its structure out to the eye of the onlooker. But no amount of light 

 will do this if its dimensions are too small 'for the crystalline lens to form 

 an image of it upon the retina. With high-power objectives the light must, 

 in the nature of things, be greatly subdued. Still, a large image, mode- 

 rately bnt properly lighted, can be far better seen than a small one many 

 times as bright. An object may in fact be too bright to be seen. If rays 

 of great angle are too powerfully converged upon it the image becomes as 

 bright as that part of the screen which represents nothing but bare glass. 

 It is in this case just like an over-exposed photograph, fiat and without 

 contrast. The image may, therefore, be too bright for the screen, just as it 



