222 



THE AMERICAN MONTHLY 



[December, 



and usually so accurate, an author 

 as Dr. Wm. B. Carpenter has fallen 

 into the error of asserting that " the 

 first really satisfactory solution of 

 the problem was that worked out 

 by MM. Nachet ; " * an error which 

 is the more remarkable in view 

 of the manner in which Riddell's 

 discovery was published and dis- 

 cussed in England, and of the use 

 made of it by the opticians of that 

 country. 



According to his own account, 

 the original binocular microscope 

 of Prof. Riddell was devised dur- 

 ing the year 1851, but it was not 

 until the following year that it was 

 actually constructed in a form avail- 

 able for use. October 2, 1852, he 

 exhibited the completed instrument 

 to the Physico-Medical Society of 

 New Orleans, and described it in 

 a communication published in the 

 New Orleans Monthly Medical 

 Register for October, 1852 (p. 4). 

 According to that communication, 

 the plan is essentially as follows: 

 " Behind the objective, and as near 

 thereto as practicable,^'' (the italics 

 are mine, J. J. W.), " the light is 

 equally divided, and bent at right 

 angles, and made to travel in oppo- 

 site directions, by means of two 

 rectangular prisms," (Fig. 32, AA'\ 

 " which are in contact by their 

 edges, that are somewhat ground 

 away. The reflected rays are re- 

 ceived at a proper distance for bin- 

 ocular vision upon two other rectan- 

 gular prisms," (Fig. 32, £B')/'sind 

 again bent at right angles, being 

 thus either completely inverted, for 

 an inverted microscope, or restored 

 to their original direction. These 

 outer prisms may be cemented to 

 the inner, by means of Canada bal- 

 sam, or left free to admit of " ad- 

 justment to suit different observers. 

 Prisms of other form., with due 



* Wm. B. Carpenter — The Microscope and 

 its Revelations, 5th ed., London, 1875, p. 60. 



arrangement, may he substituted^'' 

 (Again the italics are mine, J. J. 



Prof. Eiddell claimed that his 

 instrument was " equally applicable 

 to every grade of good lenses, from 

 Spencer's best sixteenth to a com- 

 mon three-inch magnifier," and that 

 it could be used " with or without 

 oculars or erecting eye-pieces." 

 He laid stress on the fact that a 

 true stereoscopic effect was thus 

 obtained : " It gives the observer 

 perfectly correct views, in length, 

 breadth and depth, whatever power 

 he may employ ; objects are seen 

 holding their true relative positions, 

 and wearing their real shapes ; " 

 but he laments that, " In looking at 

 solid bodies, however, depressions 

 sometimes appear as elevations, and 

 vice versa, forming a curious illu- 

 sion." October 1, 1852, Prof. 

 Riddell sent a copy of the commu- 

 nication, he was about to make to 

 the Physico-Medical Society, to the 

 American Journal of Science and 

 Arts, by which, however, it was 

 not published till January, 1853 

 (Vol. XV, 2d Series, p. 68). From 

 this it was reprinted in the Quar- 

 terly Journal of Microscopical 

 Science for April, 1853, (Vol. I, 

 p. 236). 



On account of the pseudoscopic 

 effect, which impaired the satisfac- 

 tory performance of his instrument, 

 Prof. Riddell soon acted upon his 

 suggestion that "prisms of other 

 form " might be substituted, and 

 devised an improved arrangement 

 by which that difficulty was com- 

 pletely obviated. The new instru- 

 ment was exhibited to the New 

 Orleans Physico-Medical Society, 

 April 2, 1853, and described in a 

 commimication, an abstract of which 

 was published in the New Orleans 

 Monthly Medical Register for that 

 month, (p. 78). In this instrument 

 but two prisms were used : " They 



