664 Transactions of the Society. 



magnifying power I desired for the photographs, from ten to 

 fifteen times this distance is required, and it would at first sight 

 appear necessary for this purpose to bring the objective so much 

 closer to the object than the position for which alone it is corrected, 

 that the resulting aberrations would quite destroy the definition of 

 the image. This difficulty appears to have occurred to Zeiss him- 

 self before he sent me his objectives, for he voluntarily furnished 

 with them two concave lenses said to be of 25 and 30 centimetres 

 focal length, which he directed me to screw immediately into the 

 posterior part of the brass mounting of the objectives when I used 

 them for photography, and by which he supposed the aberrations 

 introduced by distance would be corrected. One of these con- 

 caves was intended to correct the aberrations when the image 

 was projected to 1*5 or 2 metres distance ; the other was 

 expected to answer the same purpose at the distance of 3 metres 

 or upwards. 



If Zeiss consulted his distinguished adviser, Professor Abbe, as 

 to the formulae for these concaves, I do not doubt that he was 

 correctly instructed ; but if so, the lenses as actually sent do not 

 correspond very precisely with the mathematical requirements. 

 I tried them faithfully, and wrote to Zeiss as the result of my 

 examination, that when I came to study the images produced with 

 these concaves at the distances directed for each, " I became pain- 

 fully aware that a curvature of field had been introduced which did 

 not belong to the image as viewed with the 10-inch tube, while 

 great loss of definition indicated the presence of considerable 

 spherical aberration in the new combination." Photographs taken 

 under these circumstances were exceedingly unsatisfactory, and had 

 no other means of projecting images with these objectives been 

 available, I should not have succeeded in presenting a photographic 

 demonstration of their excellent qualities. 



But long before I received them I had theoretically devised a 

 method for making these projections, which on trial fully answered 

 my expectations, and gave, at any distance I chose to select, 

 images quite equal in flatness, definition, and brilhancy to the best 

 I could obtain with the 10-inch tube. This method consists in 

 placing a suitable achromatic negative lens at the end of the draw- 

 tube of the Microscope body, and slipping it by trial to the proper 

 position to bring the image to a sharp focus on the screen at the 

 distance selected, while the objective remains in precisely the same 

 focal position that was found to give the best image with the 

 lO-inch tube. The course of the rays through the objective under 

 these circumstances remains the same whether the image is pro- 

 jected to 1 or 4 metres, or any intermediate distance, and, provided 

 the concave has the requisite qualities, the sharpness of the image 

 is unimpaired. 



