PROCEEDINGS OF THE SOCIETY. 251 



metallurgists under a very great debt of gratitude, and in common with 

 the last speaker he should very much like to have an opportunity of 

 practically testing the merits of the instrument. As regards one point 

 he would like to ask for rather fuller information, and that was in 

 reference to vertical illumination under high powers such as X 1500. 

 In the instruments at present in use they got, on the whole, the best 

 definition with a cover-slip placed in the axis of the tube of the Micro- 

 scope, but the illumination was not very good. Another method was to 

 have a prism illuminator at the side of the tube, but this, although 

 improving upon the illumination given by a cover slip, was only satis- 

 factory with low powers, because the light was slightly eccentric and it 

 was very difficult to get good definition at high magnifications. If Mr. 

 Rosenhain could give them good vertical illumination with high powers 

 he had certainly achieved a brilliant success. 



Mr. Beck said there was one point about this Microscope which 

 rendered it more easy to make a flat field objective, because in this case, 

 the tube length was always the same, and the corrections could be made 

 for this given tube-length. Objectives were corrected for a certain 

 tube length, and if the Rosenhain Microscope be properly used, the 

 image would always be formed at this distance. This being so, it would 

 be possible to produce a lens which would give a flat field in one plane, 

 although there would always be a slight difference between the margin 

 and the centre, and the central definition would have to be slightly 

 impared in quality. In making comparisons of photographs as to their 

 flatness of field, it was of great importance to see that the angle of field 

 included was the same, otherwise such comparisons were valueless. 

 There always would be a limit to the flatness of the field under any 

 circumstances. In the Microscope before the meeting an iris diaphragm 

 had been placed just above the objective by which its aperture could be 

 cut down. This was frequently of great use to increase penetration or 

 depth, and to assist in cutting off internal reflection. 



Mr. Rosenhain said he was of course aware that such an instrument 

 as he was exhibiting could only be appreciated by trial, and he hoped 

 that those interested would have an opportunity of trying it ; he felt 

 convinced that they would find it much simpler in use than it might 

 appear in the description. As regards flatness of field, he thought that 

 in practice it was a difficulty that was not felt very much, provided the 

 lens were otherwise well connected and that a high eye-piece or a long 

 camera were employed. If it were really found that the want of flatness 

 was a serious matter, it could be overcome to some extent by using not 

 the approximately flat, and frequently slightly convex specimens 

 ordinarily prepared, but specimens deliberately prepared with a slightly 

 concave surface. As regards strictly vertical illumination at 1500 

 diameters, he did not know if that could be attained with anything but 

 the clear glass disk, but with the silvered half-disk illuminator a very 

 even and practically central illumination could be obtained even with 

 the highest powers, the adjustability of the illuminator especially lending 

 itself to this purpose. The ideal illumination would be with parallel 

 axial light, but when working with a Microscope objective as condenser, 

 it was impossible to obtain parallel light, owing to the spherical aber- 



