258 PROCEEDINGS OF THE SOCIETY. 



materially affected by the degree of illumination, varying according as it 

 was high or low. (He then by means of diagrams on the board further 

 discussed the point, and explained that in Mr. Nelson's table, already 

 referred to, the visible antipoint was taken to be rather less than one-fifth 

 of the breadth of the theoretical diameter of the false disc, and that this 

 result had been experimentally reached with illumination of the intensity 

 ordinarily used in high-power microscopy. As regarded the size of the 

 aperture, concerning which Mr. Beck had asked a question, he did not 

 himself inquire of Mr. Nelson what the adjustment was, but he thought 

 that probably Mr. Nelson's paper accompanying the table in the Journal 

 of the R.M.S. contained the data asked for. The two drawings appeared 

 to be made to the same scale, but the black object appeared to be just 

 a little longer than the bright one. This might probably be due to. 

 faulty draughtmanship, but, on the other hand, it would naturally happen 

 that intruding antipoints from the bright field would, to a small extent, 

 affect the visible length as well as the visible breadth of a narrow dark 

 object, and so cut it down in one direction as well as in the other. 



The thanks of the Meeting were voted to Mr. Nelson for his commu- 

 nications, and to Mr. Gordon for his remarks. 



Mr. Keith Lucas read a paper ' On a Microscope with Geometric- 

 Slides,' which he illustrated by a wooden model and by numerous photo- 

 graphs shown upon the screen. 



The Chairman, in expressing the thanks of the Society to the author 

 of this paper, said that he would no doubt be very glad to hear remarks, 

 upon it from any persons present who were so good as to offer any 



suggestions or criticisms. 



Mr. Beck said he should not like to express any opinion as to the- 

 merits of this instrument, without first having had an opportunity of 

 carefully examining it. He thought, however, that the writer was pro- 

 bably in error in supposing that the expense of making dovetailed bars 

 was much greater than that of parallel tubes, because with the machinery 

 used for the purpose dovetails could be made absolutely accurate, and at 

 a cost which was not as great as that of parallel tubes, the latter being 

 difficult to make perfectly true. He failed, however, to understand why 

 this arrangement should be called "geometric slides," as mechanical 

 equivalents of the geometric slides were in use in all directions. 



Mr. J. E. Barnard thought the subject of this paper opened up a 

 question Of great interest, for he felt sure that unless anyone had tried 

 an instrument made on the geometric slide principle, it would be im- 

 possible for him to appreciate the great advantage which it offered. He 

 had not himself applied this to the Microscope, but he had done so to a 

 table spectroscope, and he felt sure that if any maker would take up this 

 method of construction for the Microscope, it would be found of very 

 great advantage. 



Mr. Gordon said he could understand that Fellows of the Society 

 who were competent to do so would feel a little delicacy in making any 

 critical remarks upon a paper of this kind. There were, however, one or 

 two points which struck him, and upon which he should be glad to have 

 some further information. In the first place, he did not understand how 

 the coarse adjustment was held clamped in the place in which it was put. 



