1880.] 



MICROSCOPICAL jotjr:nal. 



227 



239), that he no sooner read Rid- 

 dell's article in the American Jour- 

 nal of Science and Arts (January, 

 1853), than he wrote to Nachet, the 

 well-known Parisian optician, and 

 requested him to make one for him, 

 but with a slight alteration, viz. : he 

 desired that the two lateral prisms 

 should be placed wider apart than 

 Riddell had done^ so that the in- 

 strument might be used simul- 

 taneously by two different ob- 

 servers, from which the Dutch 

 professor frankly confesses he an- 

 ticipated at that time more advan- 

 tageous results than from a stereos- 

 copic microscope. Nachet replied 

 promptly that Riddell's discovery 

 was already known to him, and that 

 it had occurred to him also that the 

 same principle might be used in 

 constructing a microscope for two 

 observers, as well as a binocular, 

 but that he thought the same ob- 

 jects could be attained in a some- 

 what different way, and for this 

 reason declined immediate compli- 

 ance with Hartings request. 



Nachet's object appears to bave 

 been in the case of the binocular to 

 avoid the pseudoscopic effect of 

 Riddell's first plan, and this in 

 point of fact he achieved, though 

 not until after Riddell had done so, 

 in a different way, for although his 

 paper " On a microscope adapted 

 for anatomical demonstrations ; and 

 on a binocular microscope" ap- 

 peared in the same number of the 

 Quarterly Journal of Microsco- 

 pical Science^ (Yol. II, 1854, p. 72,) 

 as Riddell's paper, yet this was not 

 until nine months after the per- 

 fected instniment of Riddell bad 

 been exhibited to the Physico-Me- 

 dical Society, and described in the 

 New Orleans Monthly Medical 

 Register. 



For a precise description of Na- 

 chet's plan I refer my readers to 

 his paper just cited ; the objections 



to it have been sufficiently discussed 

 by Dr. Carpenter in his work on 

 the microscope, {pp. cit.., p. 61.) It 

 certainly had the advantage over 

 Riddell's first instrument of obvi- 

 ating the objectionable pseudo- 

 scopic effect, yet it did so on Rid- 

 dell's own principle — that is by 

 means of refiecting prisms — and not 

 more efficiently than was done by 

 Riddell's second instrument, which 

 is also prior in date of construction 

 to the instrument of Nachet. 



It is clear, therefore, that Dr. 

 Carpenter's affirmation, cited at the 

 beginning of this paper, that 

 I^achet's solution of the problem of 

 stereoscopic vision with the mi- 

 croscope was the first satisfactory 

 one is true only, if Riddell's per- 

 fected instrument, which antici- 

 pated it in time, was so far inferior 

 to it in performance as to be fairly 

 styled not satisfactory. So far 

 from this being the case, however, 

 I find the Riddell instrument, now 

 in the possession of the Museum, 

 gives very satisfactory binocular 

 vision indeed, and can only sup- 

 pose that Dr. Carpenter has in 

 some way failed to become ac- 

 quainted with it, when I find him 

 in the same work, and still without 

 any reference whatever to Riddell, 

 praising the excellent performance 

 \op. cit.., pp. 64-5) of the erecting 

 binocular microscope of Mr. -1. W. 

 Stephenson, F. R. M. S., which in 

 its essential optical parts is a mere 

 copy of Riddell's second intrument. 



Mr. Stephenson's erecting binoc- 

 ular, as first made, was described by 

 that ingenious gentleman in a com- 

 munication read to the Royal Mi- 

 croscopical Society, June 8, 1870, 

 " On an erectiiig binocular micros- 

 cope," The Monthly Microscopi- 

 cal Journal., August 1, 1870, p. 

 61. It will be seen on examining 

 this paper that he obtained binocu- 

 lar vision by placing close behind 



