282 PROCEEDINGS OF THE SOCIETY. 



and hence the better appreciated, as indeed obtained in every-day prac- 

 tice when using the projecting lantern (see footnote). 



He could not conclude, however, without once more expressing his 

 congratulation to the lecturer, not only upon his photographs, but quite 

 as much if not more upon his neat and explicit way of rendering a some- 

 what recondite subject both interesting and intelligible, he felt sure, to 

 those present, many of whom were not doctors as they were themselves. 



Mr. Shillington Scales said he had found Dr. Vincent's paper of 

 exceeding interest, especially as it had been somewhat off the beaten 

 track of microscopic work such as they had at their usual meetings. 

 There were many points of clinical interest which had struck him, a 

 discussion on which, however, did not come within the scope of the 

 Microscopical Society. 



Without reflecting in any way on the beautiful photographs which 

 they had seen, he wished to endorse in every way Dr. Spitta's remarks 

 on high magnification. In his experience high power magnification 

 most emphatically proved to be a loss and not a gain to all photographs. 

 He did not consider that there was any use for a magnification of 8000 

 diameters in skilled microscopy, and it was with some surprise he had 

 heard a murmur of applause follow the exhibition of a photograph taken 

 with this enlargement. Every objective possessed a certain resolving 

 power, and needed a certain magnification to make this resolution visible, 

 but beyond this no advantage was to be gained from mere empty 

 magnification, but rather loss. He thought in this statement he would 

 be supported by every photomicrograph er present. 



Dr. Vincent thanked the members of the Microscopical Society for 

 the kind manner in which they had received his demonstration. He 

 quite agreed that magnifications at more than 2000 diameters were seldom 

 required. In some exceptional cases a magnification of 4000 diameters 



Footnote by Dr. Spitta added after the Lecture. 



The diameter of the disk of confusion is quite easily obtained ; only 

 two things are necessary : (1) to find the resolution, and (2) to multiply 

 such by the magnification. Take the first. This is obtained by Abbe's 

 law, " Multiply twice the number of waves to the inch of the light 

 employed by the numerical aperture of the objective." As, for example, 

 say the light used has 47,500 w'aves to the inch ; twice that is 95,000 ; 

 and so if the N.A. be 1'40, the resolution is 133,000, which means that 

 any two lines xWirao i n - apart can theoretically be separated if oblique 

 illumination be employed. Presuming our 1000 diameter magnification, 

 the circle of confusion is obviously T ^ ggg , equal to T ^ in. less than 

 the conventional limit. Make the magnification 2000, and immediately 

 the circle of confusion rises to double the size, and so on. Raise the 

 magnification (of course, this presumes the N.A. is not added to), and the 

 fuzziness becomes intolerable. Careful enlargement of the primary 

 negative, however, with equally careful development, will be of service 

 because, although the image is enlarged, the diffraction effects remain the 

 same ; but no further details are added, although perhaps one may be 

 able to see more on account of their increase of size. 



