20 



then be re-invented quite independently by others a long while afterwards. 

 It would perhaps be remembered that Mr. Wenham introduced the immer- 

 sion paraboloid in 1856, and that in 1871 Mr. B. D. Jackson read a paper 

 before the Club on the same subject, the re-invention in that case being by 

 Dr. Barker, but not only was its re-invention by Dr. Edmunds perfectly 

 genuine, but the instrument stands now upon what may be called a different 

 platform, because many improvements have been made in objectives and 

 methods of research. The instrument was now introduced to a more appre- 

 ciative audience than formerly, and at a time when the subject of the ex- 

 amination of minute structures rested upon a widely different footing from 

 that which existed some years ago.* 



Dr. Matthews said that, about two years ago he had come to the conclu- 

 sion that the " markings " upon Podura scale were really club-shaped pro- 

 jections from the surface of the scale, but he had never been able to de- 

 monstrate the fact as Dr. Edmunds had done to-night. He was enabled to 

 say this quite two years ago, though he knew that the idea was then thought 

 to be very heretical. He did not think that they could possibly get both 

 positive and negative images of an object by sub-stage illumination. In the 

 case of translucent objects, they might get them illuminated all round by 

 horizontal rays, but even then they would get a negative image, and they 

 could only get a positive image by light thrown down from above the level 

 of the surface of the object; all forms of illumination that did not exceed 

 the angle of 90 e from beneath must necessarily give negative images. 

 Then it must not be overlooked that, in so examining scales, they were look- 

 ing at objects consisting of two layers, with light entering from beneath. 

 They therefore got confusion between the markings on the upper and lower 

 surfaces which was much apt to mislead. As long as objects were illumi- 

 nated from beneath, he could not enforce it too strongly that they never 

 could get a positive image. 



Dr. Edmunds asked Dr. Matthews to examine the images again. 



The President said there was a class of evidence which it would be very 

 desirable to get if possible, seeing that Dr. Edmunds claimed certain advan- 

 tages and power of definition — perhaps Mr. Powell or some other gentle- 

 man with practical knowledge would give them their opinion from experi- 

 ence. 



Mr. Powell said he had not tested it upon any other objects than Amphi- 

 pleura pellucida, but he found that it showed the markings in a way which, 

 no other method would do. 



Dr. Matthews (after examination) said he saw the objects exactly as Dr. 

 Edmunds had described them, but it was quite clear that the illuminating 

 rays approached them from beneath, and that in order to get a positive 

 image the light must fall upon the object from above. 



Dr. Edmunds thought there were reasons why he might still ask Dr. 



* Note. — Upon the question of priority, Dr. Edmunds writes :— " As between Mr. 

 Wenham and myself, I need only to observe that the paraboloid described by Mr. Weuham 

 was not like miue, either in its formula or in its effects, and that the methods of working 

 and applying the two instruments as accessories to the microscope dj not in the least 

 resemble each other."— Ed. J. Q. M. C. 



