1018 SUMMAKY OF CUERENT RESEARCHES RELATING TO 



brought nearer to the condenser. This movement is effected by a sliding 

 motion of the stand K between two wooden guides, by means of rackwork. 



Between the two condensing lenses it is necessary to insert a glass 

 trough S, with plane sides filled with concentrated alum solution, to prevent 

 the over-heating of the object. Thick or dark-coloured objects are very 

 easily over-heated ; an energetic and invariably sufficient means of cooling 

 is obtained by a current of air directed upon the surface of the object or 

 upon the cover-glass. Compressed air is obtained from a loaded india- 

 rubber bag (above A). The delivery tube is of brass, with an aperture of 

 1/2-1 mm., and is fixed to the stand at an angle of 45° on the under side 

 of the objective ; the distance of the aperture from the cover-glass being 

 about 1 to 1^ cm. 



The coarse-adjustment is effected by the rackwork on P, the fine- 

 adjustment by the micrometer-screw T. The objects are held by one or two 

 of the usual clips against the vertical stage. 



The nearer the lamp is to the white paper screen, the brighter will be 

 the images, but the less the amplification. After several trials a distance 

 of 5 metres betw^een the object and the screen has proved the most con- 

 venient. By using a stronger source of light, this distance may easily be 

 increased to 6-10 metres. 



To bring the projected image as near as possible to my audience, I 

 place the electric lamp in the middle of the amphitheatre, and the screen in 

 front of the fi.rst row of seats, an open passage being left between the lamp 

 and screen. There is no objection to the image being seen obliquely fore- 

 shortened by those of the audience who are at the sides ; it scarcely loses 

 in clearness thereby. 



White paper does not give nearly such bright and clear images as a 

 plaster surface. This is made by bending an iron band into a circular or 

 rectangular form, making a network of wire across it, and placing the whole 

 upon a glass plate which has been rubbed over with powdered talc. 

 Alabaster plaster is poured upon the netwrork, and when it is cool the 

 whole mass is lifted off. The projection plate should have a diameter of 

 1 • 2 to 2 metres. Trials with transparent screens, such as oil paper, tracing 

 paper, or ground glass plates gave unsatisfactory results. 



After numerous experiments it has been found that the finest images 

 are given by those objectives which have been made for a long tube, 

 especially the so-called photographic objectives. It is not advisable to 

 make use of an eye-piece for projection purposes. 



To cut off all extraneous light, it is a good plan to place over the 

 condensing lens Hi and the alum trough S, a light cardboard case which is 

 prolonged into a cardboard tube towards the stage of the Microscope in the 

 direction of the beam of light. 



Finally it may be mentioned that it is possible to use a horizontal 

 stage. The beam of light is then reflected upwards by the ordinary plane 

 mirror, and again deflected into a horizontal direction by a prism of flint 

 glass, which rests against the upper nose-piece aperture. 



Of the objectives which I have employed, the following give the best 

 defined images : — Hartnack, objectives 1 and 2 ; Seibert, 1 in., 1/2 in., and 

 1/1 in. photographic objectives ; Winkel, objective 7 ; as well as water- and 

 oil-immersion objectives of various makers. 



Absolutely colourless images of extraordinary clearness are given by the 

 combination of the new Zeiss apochromatic objectives with the correspond- 

 ing ' projection-eye-pieces.' Though this combination is unrivalled for 

 photographic purposes, it is not convenient for demonstration, since the 

 image is too faint and of too limited dimensions." 



The whole apparatus is supplied in this country by Mr. K. Schall, of 



