PROCEEDINGS OP THE SOCIETY. 1113 



whicla he said answered the purpose very well. The plan was to 

 place behind the objective a piece of cover glass of the same thickness 

 as the one upon the slide, and if the (me corresponded precisely to 

 the other the aberration would be corrected approximately as by the 

 screw-collar. 



Dr. Anthony thought this might be of some use where a person 

 always used cover-glasses of a uniform thickness ; he adopted this 

 course himself, always using those of a thickness of • 005. But if they 

 had a number of objects by various mounters, how were they to deal 

 with the matter, not knowing what thickness was used ? He should 

 be glad to hear what the President thought of the idea. 



The President said he thought the idea was excellent in principle, 

 but open to great difficulty in practice, as the correcting glass must 

 continually be altered according to the thickness of the cover. He 

 was afraid that it could not be regarded as a practical method except 

 for very special cases. 



Dr. Matthews also considered that the plan would be unsuitable 

 for general use as they were now-a-days called upon to deal with 

 covers of unknown thickness. 



Mr. J. Mayall said that the cover-glasses at one time were marked 

 according to their thickness "005, "006, &c 



Mr. J. Mayall, jun., described Eiddell's Binocular Microscope, 

 which was exhibited by Mr. Crisp, and was of considerable interest as 

 being a duplicate of Prof. Eiddell's original Microscope, which now 

 belonged to the Army Medical Museum at Washington {supra, p. 1059). 

 He pointed out as a noteworthy feature that it was provided with a 

 means of separating the prisms so as to give to each eye-piece a full 

 field of view. There was also a screw with a right- and left-handed 

 thread for separating the tubes to suit the width of the observer's 

 eyes. An ingenious application of reflectors at the top of the eye- 

 pieces effected a perfect inversion of the image, so that the instrument 

 could be used for dissecting purposes. He had tried a few experi- 

 ments with it, and had found its performance to be very fair. There 

 were some inconveniences which might be improved upon with ad- 

 vantage, such as the rotation of the prisms above the eye-pieces, so 

 that it was not easy to prevent them from getting shifted and causing 

 a confusion of the images. There was also too little room for the 

 nose between the tubes, and during a protracted observation the 

 breath condensed upon them to a great extent. There was no fine 

 adjustment, but ho believed the Microscope was only intended to be 

 used with low powers. It was, he thought, an instrument of great 

 interest as having been made so early as 1853. lie hoped that 

 English makers would take up the point of providing some means of 

 separating the prisms, which offered such advantages that he won- 

 dered a similar method had not been adopted hitlierto. Prior to this 

 instrument being made Prof. Riddell had devised another, also 

 described in the Society's Journal — tlie fonn afterwards adopted by M. 

 Nachet. It was, ho thought, a jjoiiit of special interest in the history 

 of the development of tlic rnodoru Jiinocular Microscope, that so early 



