297 



it would be distorted towards the edges, and the corrections 

 given must be applied in order to reduce it to accuracy. The 

 amount of the distortion was, however, not very serious, being 

 only five per cent, in a drawing of 8in. in diameter, and two 

 per cent, in one of 5in., but in 2|in. the distortion was so slight 

 as to be practically nil. 



Mr. Tngpen inquired if Mr. Nelson had ever tried to remedy 

 the distortion by an adjustment of the eyepiece ? If they took 

 two plane convex lenses and placed them back to back it would 

 be possible to shift them so as to get an image which would 

 hardly be distorted at all. 



Mr. Nelson said he had worked at something of this sort, but 

 did not do it in the same way, although his idea was similar. 

 He put a lens below the camera to correct the distortion in such 

 a way that whilst it had no magnifying power it distorted the 

 margins. 



Mr. Ingpen said that in the course of some experiments on 

 eyepieces he found out the variations which could be produced, 

 and he thought the principle could very likely be applied to 

 meet the case by the exercise of Mr. Nelson's well-known skill. 



Mr. Enock inquired how a Camera lucida could be fitted over 

 the eyepiece of such a pattern as the one on the table ? 



Mr. Nelson said it could be done by Zeiss's plan by passing 

 it over the rim and fixing it with three set screws, which he 

 thought was a very clumsy method. 



Mr. Enock thought so too. Certainly it was a method which 

 tried the temper very much. 



Mr. Karop read a paper " On the Illumination of Objects 

 with Artificial Light under Low Powers." 



Mr. Ingpen said that with higher powers it was easy to use 

 the whole aperture of the object glass, but the great advantage 

 of this plan was that it enabled the whole aperture of a low 

 power lens to be utilised. 



Mr. Nelson thought the point brought out by this paper was 

 an extremely valuable one, especially for low and medium 

 powers. 



The President said their Secretary was always ready to come 

 forward and help in any emergency, and they had now to 

 thank him not only for doing this, but also for the very excel- 

 lent paper he had given them. 



Joukn. Q. M. 0., Series II., No. 39. 21 



