530 PROCEEDINGS OF THE SOCIETY. 



not to take negatives upon a largo plate, hut on a quarter-plate first and 

 afterwards to enlarge the pictures from the original negatives. Speci- 

 mens of such enlargements were also exhibited enlarged about nine 

 times from the originals. The great advantage of this method was in 

 the amount of light gained for the purpose of focusing. If a good 

 sharp picture was produced, the gelatin plate would admit of consider- 

 able enlargement up to the point where the grain began to show. This 

 quarter-plate size was also the proper one for lantern slides, which were 

 so much in request at the present time for purposes of demonstration. 

 In the ordinary forms of stand the diaphragm plate is placed immediately 

 below the stage ; but, for photographic purposes, this was, in his ex- 

 perience, entirely useless, because it only cut off the edge of the field 

 without either improving definition or correcting spherical aberration. 

 In practice, he had found that by removing the diaphragm plate a 

 certain distance from the object it then ceased to cut off the field and 

 began to reduce the light and to improve the penetration and definition. 

 Opticians, he knew, were inclined to doubt whether this arrangement 

 would do what he claimed for it ; but he could only say that, with a good 

 light he could easily show that such was the fact. In cases where high 

 powers were used this answered very well ; but it would not work, how- 

 ever, with low powers unless the diaphragm plate was removed to a 

 distance too great to be convenient in practice. He had now, therefore, 

 devised the plan of introducing a short 1^ in. condenser behind the 

 stage, and about 3 in. in front of the diaphragm plate, in this way throw- 

 ing it out of focus. The effect of this was that the same improvement 

 in penetration and definition was obtained, but on a much shorter dis- 

 tance. The use of the diaphragm was of the utmost importance in 

 photography where the most perfect focusing and definition were 

 required. Attention was also called to a method of clamping the object 

 in position when the focus had been obtained ; also to a plan for 

 obtaining a fine-adjustment by means of a tangent screw. 



Mr. Beck said he did not usually like criticizing matters of that 

 sort, because he was one of those who had great doubts as to the value 

 of photographic images produced by the Microscope. Photography 

 might produce what was seen by the eye ; but in many cases they were 

 frightful distortions. If they looked at the photograph shown of the 

 proboscis of the blow-fly, they would see that it showed every hair as 

 being double, an effect which he considered was due to the removal of 

 the diaphragm having caused distortion by diffraction. If a diaphragm 

 was of any use at all it was to cut off certain rays which caused indis- 

 tinctness of focus, or to cut off the central rays, so that the circumferen- 

 tial rays could be used alone, and the object in photography should be 

 to get rid of all those inaccuracies which a diaphragm, when it was 

 properly used, would get rid of. He did not wish to criticize the 

 apparatus before them, which seemed to be beautifully made, except that 

 he thought there was rather an inconvenient distance to stretch out in 

 order to reach the focusing screw. 



Mr. Teasdale said he had practised photography more or less for the 

 last thirty years, and had never found it necessary to pay the slightest 

 regard to the diaphragm, although he might have occasionally used a 

 temporary one. In the case of photomicrography, the place for it was 

 certainly behind the lens. The chief difficulty in focusing was due to 

 want of light. Focusing should always be done with as much light as 



