230 



THE AMEEICAN MONTHLY 



[December, 



of Riddell's first paper, that a bin- 

 ocular microscope is " by no means 

 a novelty, and its invention dates 

 nearly two centuries back." The 

 true history of these early attempts 

 will be found in the work of Hart- 

 mg{op. cit., Bd. Ill, S. 101 etseg.), 

 from which it would appear that 

 the binocular microscope of Cheru- 

 bin (1677) was preceded by one 

 invented by Lippershey (1609), and 

 another by Antonius Maria de 

 Reita (1645), and followed by others 

 described in the works of Zahn (2d 

 ed., 1702), and Bion (3d ed., 1726). 

 Harting has correctly pointed out 

 {op. cit, Bd. III., S. 239), that the 

 plan employed in all these earlier 

 attempts was simply to fasten to- 

 gether two separate microscopes in 

 such a way that they could be 

 looked through simultaneously with 

 both eyes ; a plan wliich was neces • 

 sarily limited in its application to 

 very low powers, even when the 

 adjoining lateral portions of the 

 two object glasses were cut away so 

 as to allow their closer juxtaposi- 

 tion, as was done by Cherubin. 



With these earlier efforts, there- 

 fore, the discovery of Riddell has 

 nothing in common. He undoubt- 

 edly deserves the credit of having 

 discovered and first published the 

 optical principle, on which all the 

 most successful binoculars made 

 prior to the present year depend. 

 He first showed that the cone of 

 rays proceeding from a single ob- 

 jective may be so divided by means 

 of reflecting prisms, placed as close 

 behind the posterior combination of 

 the objective as possible, that or- 

 thoscopic binocular vision can be 

 obtained both with the simple and 

 the compound microscope ; and this 

 principle, whether carried out as he 

 himself did, or in the slightly 

 modified manner adopted by some 

 of those who have attempted to 

 improve upon it, has been until the 



present year the only plan upon 

 which really satisfactory binocular 

 microscopes have been constructed. 

 The remarkable paper recently 

 published by Prof. E. Abbe of Jena 

 ("Beschreibungeines neuen Stereo- 

 skopischen Oculars," Zeitschrift 

 fur MikrosTcopie, 2te Jahrg., Heft 

 8, 1880, S. 207 ; also this Journal, 

 p. 201), marks a new era in the his- 

 tory of the binocular microscope. 

 Abbe secures binocular, but not 

 stereoscopic vision, by means of re- 

 flecting prisms, and then obtains 

 the stereoscopic effect, which can 

 be made at pleasure either ortho- 

 scopic or pseudoscopic, by merely 

 cutting off with suitable diaphragms 

 the outer or inner halves of the 

 cone of rays above the eye-lens of 

 each eye-piece. I have every rea- 

 son to believe that his instrument 

 (which I have not yet had the pleas- 

 ure to see, although I have sent for 

 it) as manufactured by Carl Zeiss 

 of Jena, will be an improvement 

 upon the binoculars at present in 

 use with the compound microscope. 

 But, however this may turn out, the 

 arrangement devised by Riddell for 

 the dissecting microscope can hard- 

 ly be superseded ; and even with 

 the compound microscope his plan, 

 or some of its modifications, will 

 probably continue to be employed 

 to a certain extent. 



Cercaria Hyalocauda.— Hald. 



BY HERMANN C. EVARTS, M. D. 



After having kept the common 

 pond snail {Physa heterostropha, 

 Say) for several days in a shallow 

 dish, containing water, I observed 

 the above-mentioned larval form of 

 a Trematode. 



These cercaria actually came from 

 the snail, and in large numbers, but 

 only for two or three hours in the 

 afternoons, when the sun was shin- 



