154 



Mr. C. Beck, in reply to the President, said that their objectives were 

 always tested by focussing upon a podura scale, and altering the position of 

 the scale so as to examine it in all directions. It was very easy to get an 

 objective which would show the scale very well in one position, but not in 

 others also. 



Mr. Nelson said that perfect accuracy could not be obtained in that way, 

 because as the light did not proceed from a point, the cone of light was not 

 really a cone, as they would find if they tried the image of the flame of a 

 lamp end-ways or flat-way ; if in the former case the definition was good 

 they would find that there would be in the latter case a loss of intensity 

 which would spoil the definition. 



Mr. Beck said that they endeavoured to get it true for central light by 

 having a cone of light accurately centred. 



Mr. Michael said it was clear to him that Mr. Nelson was right in saying 

 that turning the podure scala was not by any means the same thing as 

 turning the objective ; to make it so it would be necessary to turn the light 

 also, otherwise the test-object would be viewed under different conditions 

 in different positions. Though they might have central light to the objec- 

 tive all the time, it might not be also central light to the object. 



The thanks of the meeting were voted to Mr. Nelson for his communica- 

 tion. 



Mr. C. Beck exhibited and described a new serial section cutting machine, 

 designed to meet the want of some reliable instrument which would cut 

 any number of consecutive sections of a specimen and preserve them in 

 exactly the same order in which they were cut. The importance of being 

 able to do this was very great in many kinds of investigation, and the in- 

 strument exhibited was designed to do all the work of the Cambridge 

 machine, but at considerably less cost, It was not yet perfect, but still it 

 did its work so well as to merit attention. Sections could be cut of various 

 thicknesses, and accessories for freezing were supplied with it. 



Announcements of meetings, &c, for the ensuing month were then 

 made, and the proceedings terminated in the usual manner by a Con- 

 versazione, at which the following objects were exhibited : — 



Ichneumon fly ... Mr. F. W. Andrew. 



V. S. cochlea of a kitten Mr. E. C. Bousfield. 



Diatoms, Mhabdonema arcuatum (in conjugation) Mr. T. H. Buffham. 



Scale of Lady-fish from Bombay ... ... ... Mr. H. Epps. 



Section of Coniferous Wood (fossil) ... ... Mr. W. M. Holmes. 



Epidermis of mistletoe ... ... ... ... Mr. G. E. Mainland. 



Diaptomus Westwoodii ... ... ... ... Mr. R. T. G. Nevina. 



CIvelifer museorum ... ... ... ... ... Mr. A. C. Tipple. 



Attendance — Members, 64 ; Visitors, 6. 



