360 



edge of the field was produced, and suggesting some plans to 

 modify or remedy it. One of these was by the use of a thick 

 plate of glass, which would, by refracting the rays, cause a 

 greater displacement of the image at the periphery than at the 

 centre of the field, but for other reasons the idea was abandoned. 



Mr. Hardy was sure the members of the Club would be very 

 much indebted to Mr. Nelson for letting them know just what 

 the error was ; but the mention of the subject brought to his 

 mind a method of drawing which he had used himself, and 

 thought would be found a very good one. When a person was 

 using the microscope and wanted to draw an object he of 

 course wanted to draw it as it was seen, and it required a great 

 deal of practice to draw direct from the microscope ; but by 

 using a neutral tint reflector and turning it at right angles they 

 would find it corrected the false views, and that if the drawing 

 board was placed parallel to the reflector there must be very 

 little error indeed; indeed he doubted very much if there 

 would be any error at all. Mr. Hardy explained further what 

 he meant by a drawing upon the board. 



Mr. Nelson pointed out that they must always have the same 

 error in whatever position the drawing-board was placed, be- 

 cause they were projecting a portion of a sphere upon a plane 

 surface, and this surface always formed a tangent to the arc. 

 The problem to be solved was how correctly to project part of 

 a spherical image upon a plane. By a further diagram he 

 showed that the plan suggested by Mr. Hardy did not in any 

 way alter the conditions. 



Mr. Morland asked if Mr. Nelson's calculations of error were 

 based upon the assumption of a distance of ten inches ? 



Mr. Nelson said that was so. 



Mr. Hardy thought that if an apian atic lens was used there 

 would be very little distortion to correct ; but apart from this 

 there was really so little curve that there would be very little 

 error to be corrected. 



Mr. Nelson said that Mr. Hardy's supposition was quite 

 erroneous, and that aplanatism had nothing whatever to do 

 with the subject. Mr. Hardy by his method did not remove 

 any portion of the error, but only altered its position. 



Mr. Ingpen said if they took a drawing of an object ruled in 

 squares the whole of the error would be at once made clear. 



