326 PROCEEDINGS OF SOCIETIES. ["™rm.^, JUS^Pn^^a ' 



duced by Dr. Pigott between the eye-inece and the object-glass, the 

 image is as clear, bright, and definite as when the flame is seen with 

 the naked eye. This he considered a very satisfactory proof of the 

 diminution of spherical aberration. It was known that he (the Presi- 

 dent) doubted at first the accuracy of Dr. Pigott's interpretation of 

 the Podiira scale; and that he could not feel j^erfectly convinced 

 until he had satisfied himself that the representations were those of 

 the true test-scale, by examining some of his own test-objects by Dr. 

 Pigott's method. He now, however, believed that Dr. Pigott had 

 conclusively jDroved that the beads so distinctly visible under Dr. 

 Pigott's arrangement are characteristics of the scale. Ho (the 

 President) thought that the method of illumination employed by 

 Dr. Woodward could hardly lead to a correct representation, as had 

 been stated by Mr. Slack. Dr. Pigott's whole arrangement is totally 

 difierent, and leaves the impression on the mind that what the eye has 

 Been does in reality exist. 



Mr. Beck said the remark made with regard to the want of true 

 sphericity in globules of mercury had no force as against his advocacy 

 of their use for correcting aberration. The value of the test consisted 

 in the ease with which a simple globule could be se^jarated into 

 minuter globules, each of which had the cax^acity of reflecting a point 

 of light. 



The President said that small points of light used by Dr. Pigott 

 were equivalent to small globules of quicksilver. 



Mr. Lee then read a paper of Dr. Bowerbank's, of which notice 

 had been given at the previous meeting. 



The thanks of the meeting were presented to Dr. Bowerbank. 



Mr. Holmes then read a paper describing an invention of his own, 

 by which, through the use of two object-glasses, or two halves of 

 object-glasses, a binocular and stereoscopic view of ojjaque and traps- 

 parent objects can be obtained ; and also a means of producing, by 

 certain mechanical appliances, such motion of the optical parts of the 

 instrument, and such combinations, as shall secure at the will of the 

 operator either binocular or monocular vision. 



After some time had been devoted to the examination of the 

 instriunents exhibited by Mr. Holmes, Mr. Brooke said he thought 

 that it would be a matter of extreme difficulty to obtain the necessary 

 accuracy of centering with halves of lenses only, and that it would be 

 impossible with high powers, say with anything above |-inch, with 

 halves of lenses only to give a good definition. 



Mr. Slack believed that Mr. Holmes had greatly exaggerated the 

 defects of definition with the Wenham prisms. A first-rate instrument 

 showed difficult test-objects nearly as well when the prism was used as 

 without it. It also appeared to him that optical errors of dijffraction 

 must be introduced by the plan of using halves of lenses. He doubted 

 the possibility of effecting the perfect division and centering of high 

 powers. 



After some further discussion on this subject, the President said 

 that it appeared to be the general imiiression that Mr. Holmes had 

 produced an instrument capable of giving a jjerfectly stereoscopic 



