CORRESPONDING SOCIETIES. 313 



image to be of a size, brightness, and definition sufficient to render it visible 

 to an audience. In most of the earlier apparatus the object was illuminated by 

 limelight concentrated on it by an ordinary condenser, and was placed upside 

 down and vertical, so as to be parallel to the screen, on which the image was 

 projected by a lens, with its axis horizontal. This arrangement gave an image 

 which was correct when viewed through a semi-transparent screen, so that the 

 lantern had to be behind the screen. If it were placed in front in the usual 

 way the image could be obtained right way up by the ordinary method of in- 

 verting the object, but in that case it was inverted from side to side, like the 

 image of a person as seen by himself in a looking-glass. This rendered labels 

 and all printed and written descriptions illegible, and made the apparatus 

 useless for showing diagrams and figures in books. This difficulty can be got 

 over by reflecting the rays on their way to the screen by a plane mirror, which 

 produces a similar error in the other meridian, so that th« whole can be set 

 right by altering the position of the object. The optic axis of the lens can 

 also be kept vertical, and the object lies flat, which is a great advantage for 

 small, loose objects on trays, such as fossils, or coins and medals. 



I employed this device in an instrument I devised and had constructed in 

 1905-06 before the British Association Meeting at York. A lantern was required 

 to project photographic slides on a very large screen so as to be visible to an 

 audience of, say, 1,500 or 2,000 at the evening lectures. It was decided to 

 make one with every item the best possible. I had an old Dalmeyer portrait 

 lens that was very suitable for the front projection lens, and it was found on 

 inquiry and experiment that a condenser with a meniscus as the back lens gave 

 the best results. Such an one was procured, and this completed the optical 

 part of the outfit. It remained to obtain an electric lamp that would give with 

 the 2O0-volt town supply the most powerful and steady light available. Mr. 

 Hame, of the Corporation Electric Department, and his junior, Mr. Foster, 

 were at much trouble in getting such an one which had been found reliable. 

 It had a hand feed. These components were all plotted in their places on paper 

 and a lantern drawn round them. The lantern was made by a local joiner and 

 worked perfectly the first time it was tried. It is the best and most powerful one 

 I have seen, except perhaps that at the Alpine Club in London, where the 

 different components were selected separately by various experts in a similar 

 manner. 



I tried to combine an apparatus for opaque projection with this by using 

 the very powerful horizontal beam of light issuing from the condenser and 

 turning it down on the opaque object by a plane mirror. The light, as it 

 came off from the object, was collected by a rapid portrait lens, placed vertically, 

 and reflected on to the screen by a mirror as described above. The image 

 appeared right way up, right side forward, and of suitable size, and the result 

 was thus satisfactory as showing that our optical calculations were correct ; but 

 the loss of light by reflection from two mirrors and from the object, besides 

 that by passing through several pieces of glass in the condensers and lenses, 

 was so great that the attempt was abandoned for the time. 



It will be remembered that the voltage necessary for a single arc light is 

 only 40 to 50, and if a current of a higher pressure be used, which in our case, is 

 220 volts, it is necessary to introduce a resistance which reduces the amount- 

 that can pass, and converts the surplus energy into heat which is wasted. 



In the spring of this year I saw an account of a new lamp invented by 

 Mr. Beardmore, and made by Marion & Co., which would work four arc- 

 lights in series with one current, used four times over ; only a very small 

 resistance was thus required to steady the flow, so that practically all the 

 power was utilised. The arcs being arranged in close proximity give a very 

 brilliant light without a condenser, and a concave white screen behind utilises 

 much light that would otherwise be wasted. 



Mr. Hame was again kind enough to order the proper size of lamp, while 

 Mr. Foster again superintended the practical fitting. The result is that we 

 get four powerful arcs almost close to the object, which is thus intensely 

 lighted, and the light from it is taken up as before by the lens and mirror and 

 focussed on the screen. No advantage was found by the introduction of con- 

 densers, which, moreover, rendered the light streaky and uneven. 



