329 



which perfect spherical correction only is aimed at. Such a lens 

 corrected for A. 5461 would be of the greatest use in photography, 

 and should be a cheap article, and possibly better than an 

 apochromat, owing to its flatter field. Thanks were expressed 

 to Messrs. Wratten & Wainwright, Ltd., for the loan of a 

 set of special mercury-lamp monochromatic filters, and to Mr. 

 H. F. Angus for the loan of several spectroscopes provided for 

 the examination of the spectrum. 



On the motion of the President a vote of thanks was passed 

 to Mr. Banfield for his exhibit. 



A paper on " Dark-ground Illumination," communicated by Mr. 

 E. M. Nelson, F.R.M.S., was then read by the Hon. Secretary. 



At Mr. Nelson's request, ]\Ir. C. Lees Curties, F.R.M.S., 

 exhibited a specimen of the lamp and condenser described. The 

 action of the substage condenser when used for dark-ground 

 illumination was very well shown. A glass cell containing a 

 fluorescing solution was placed on the stage above the condenser, 

 and the emergence of the oblique rays from the top lens and 

 their passing upwards through the object and divergence out- 

 wards away from the front lens of the objective conveyed to 

 members a very much better idea of "how it worked" than any 

 number of repetitions of diagrams or explanations. 



Mr. W. Traviss exhibited several objectives fitted with an iris 

 immediately behind the back lens. The rotating collar actuating 

 the iris was graduated to show the N.A. employed. 



The Hon. Secretary ( Mr. W. B. Stokes) expressed his appreciation 

 of Mr. Nelson's paper, which gave instructions so badly needed. 

 He then warned members of the possibility of reducing the 

 aperture of the substage condenser in one direction if the pro- 

 portion between the width of the source of light and the width 

 of the back lens of the substage did not equal the proportion 

 between the focal length of the bull's-eye and the distance of 

 the latter from the substage condenser. He assumed that the 

 image of the bull's-eye was focused upon the object. Neglect 

 of the suggested equation only affected the centre of the field 

 when the edge of a lamp flame was the light-source. To avoid 

 too great a distance between bull's-eye and condenser, he sug- 

 gested shortening the focal length of bull's-eyes, which might 

 necessitate the use of fused silica for the latter when intended 

 for use with a Nernst lamp. 



