88 INST. -COM. M. A. AINSLIE, DR. E. C. BOUSFIELD 



bound to say that the English and other opticians I have had the 

 pleasure of dealing with have put these lenses at my disposal without 

 stint, but I feel that we want to strike a far higher average of 

 excellence. We do not want 95 per cent, perfection in 10 per cent, 

 of the cases, and the remainder under 60 per cent. We want to 

 strike a 90 per cent, average and depend upon it. I want to mention 

 that point because of apochromatic objectives which I have seen 

 made by English manufacturers. I can single out a 4 mm. of Mr. 

 Swift's, a 4 mm. .85 aperture of I\Ir. Watson Baker, and a 2 mm. 

 of Mr. Watson Baker. I cannot tell you much about the latter, 

 because I do not know what became of the lens, but it was a very 

 perfect one indeed. I can fully echo any remarks that have been 

 made as to the high quality of possible w^ork of English opticians, 

 but I also should like to mention that I wish they would always 

 do it. 



Dr. E. C. Bousfield : The few remarks that I shall make to-night 

 have been prompted by what has already fallen, especially from 

 Mr. Barnard, with regard to apochromatic lenses. I think perhaps 

 my experience of them is longer than that of anyone here, since, in 

 conjunction with my friend, the late Mr. Lees Curties, whose loss 

 so many of us deplore, the first photographs made in this country 

 with Zeiss apochromatic lenses were made in my own house, on a 

 dining-room table, incidentally with the tunnel built up with books 

 between the microscope and the camera, and the result was perfectly 

 satisfactory. I think success in this matter depends comparatively 

 little upon the brasswork, but a great deal upon the glasswork, and 

 almost most of all upon the operator. The apochromatic lenses 

 which were at first supplied were of the finest possible quality. I 

 think I have seen nothing better than the first 2 mm. apochromatic 

 lens which I had from Zeiss, but, unfortunately, as was the case 

 with all these early lenses, the glass was very soon attacked by the 

 atmosphere, and in substituting a glass which was more resistent, 

 the qualities of the lens suffered very considerably, and when it 

 was returned to me the field was very much less flat than it had been 

 in the first instance. There is one maker who has not been referred 

 to to-night, but who was absolutely, I believe, the pioneer of apo- 

 chromatic lens work in this country — I mean the firm of Powell 

 and Leeland. Certainly they turned out — and I say it without any 

 disrespect to anyone else- —the very finest work in the shape of glass- 

 work that has ever been used in the world, and British glass-work 

 has been of remarkable excellence. They supplied me, for trial, with 

 an apochromatic lens of their own manufacture, which was calculated 

 in England and made in England, and it was absolutely perfect, 

 but it had the same fault that the Zeiss lenses had, in being made 

 of unstable glass. None of the 2 mm. lenses that I have seen made 

 of the more resistent glass are at all free from roundness of field. I 

 notice that in one of the papers that is to be laid before us reference 

 is made to this roundness of field, and in actual working, those of 

 us who have tried it with, say, 1,000 diameters, will agree that it 

 is a very serious trouble indeed, and I do not see any way of getting 

 over it. Lower magnification and a longer camera does not do so. 

 T suppose the reasons are mathematical ones, which are beyond me. 



