152 



The thanks of the Club were voted to the donors. 



The following gentlemen were ballotted for and duly elected members of the 

 Club :— Mr. George Hy. Baker, Mr. Fredk. C. Barnett, Mr. A. W. Chapman, Mr. 

 Alfred E. Haddou, Mr. J. W. Jenkins, Mr. George James Jones, Mr. Francis J. 

 Kittsett, Mr. A. F. Mayhew, Mr. Charles Mills, Mr. William L. Smith, Mr. 

 William A. B. Williams. 



Mr. Ingpen said that he feared he had no improvements to bring forward, nor 

 could he even promise anything new, but he thought that a gossip about eye- 

 pieces might not be quite uninteresting or useless. The early histoi-y of the eye- 

 piece was connected with that of the tt-lescope, the invention of which was due to 

 the discovery that the inverted diminished image formed in the focus of one lens 

 could be magnified by another lens, and this principle was not applied to the mi- 

 croscope till long aftei'wards. The telescope and the microscope were, however, 

 but modifications of the same instrument, and if an object-glass and eye-piece of 

 equal foci were employed, a telescope or mici'oscope of no magnifying power 

 would result. This form was used in Martin's " Graphical Perspective :" a net- 

 work of lines ruled in squares on talc was placed in the common focus of the 

 lenses, and the objects shewn in each square copied on similar squai'es ruled on 

 paper. There was also a "simple" telescope, where the eye, placed within the 

 focus of the object-glass, became itself the eye-piece as in the simple microscope. 

 This had been described by Dr. Dick. The early eyepieces were either single 

 convex or concave lenses, the former being placed outside the focus of the object 

 glass, the latter within the focus. In the first case a real image of the object 

 was formed, which could be received upon a screen, and the eyepiece was called 

 " positive," in the latter there was no real image, but the rays of light from the 

 object glass were rendered parallel, and so received on the eye as an enlarged 

 object. This was called a " negative" eyepiece. This term was afterwards ap- 

 plied to the Huyghenian form, where the real image was formed between the 

 lenses. The Huyghenian eyepiece was perhaps the most valuable accessory to 

 optical science ever invented. Huyghens used it merely to distribute the 

 spherical distortion between two lenses, and so flatten the field of view, but it 

 was also found to correct the residual chromatic aberrations of the achromatic 

 object glass, by delicate adjustment of the distance between the lenses. The 

 field of view is nearly flat, but generally somewhat concave, so that parallel lines 

 on a micrometer appear to diverge slightly from each other at the margin of the 

 field, while lines forming a diameter appear straight. This efi'ect varies accord- 

 ing to the objective, the length of body, and the focal length of the observer's 

 eye. and the field of view can be made concave, convex, or sensibly flat by care- 

 ful adjustment of the distance between the lenses of the eyepiece. Various modi- 

 fications of the Huyghenian eyepiece had from time to time been suggested and 

 constructed. One of the earliest was the use of crossed lenses (i.e. double convex 

 lenses of very unequal curvatures, generally 16) instead of plano-convex lenses. 

 This arose partly from the difliculty of producing lenses with flat surfaces, par- 

 ticularly at the edges, and rendered the centreing easier. Then there was the 

 form suggested by Professor Airy (Camb. Phil. Trans. III. i. 61), the field glass 

 being meniscus (11.4), the eyeglass a crossedlens (1.6). Of other eyepieces used 

 for the microscope the "Kellner" forni with the focal lengths of the lenses in the 

 proportion of 1.2. instead of 1.3. as in the Huyghenian, was often met with in 

 Continental microscopes. In some of these the field lens was placed in the focus 

 of the eye lens. These seemed to act pretty well with short bodies, but to be on 



