30 



THE AMEPJCAN MOIS-THLY 



[February, 



following respect, viz. : that his (Mr. 

 Stephenson's) prisms were so placed, 

 that, combined with the reflecting 

 above, they acted as an erecting 

 instrument, and by entering into 

 the cell of the object-glass could be 

 used for high powers whenever re- 

 quired ; whilst those of Professor 

 Riddell were placed above the ob- 

 ject-glass simply, to produce bino- 

 cular effect. He had only just heard 

 of this through his friend Mr. Crisp, 

 and he took the earliest opportunity 

 of notifying it to the Fellows of the 

 Society." 



It is not surprising that I was 

 ignorant of what had taken place se- 

 venteen years previously, as the mi- 

 croscope was a comparatively new 

 instrument in the year 1870 ; but it is 

 surprising that Thomas Ross, by 

 whom my instrument was constructed 

 was also unaware of the fact, as he 

 pressed me strongly to allow him to 

 patent it at his own expense, thus 

 conclusively showing that he had no 

 idea that the light had been so divided 

 before. 



If Colonel Woodward read my 

 disclaimer at that time, he will, I am 

 sure, regret that he had forgotten it 

 when writing his paper to which I 

 have referred. 



My reinvention of the instrument 

 arose as follows : I was at Avork on 

 the Polycystina and could never over- 

 come the difficulty of picking out 

 new, or rare species in consequence 

 of the inversion of the image ; this 

 induced me to apply myself to some 

 device by which the difficulty could 

 be overcome. I soon saw that if a 

 binocular could be constructed with 

 one reflection in each plane, the ob- 

 ject would be accomplished — this, 

 almost of necessity, suggested the re- 

 flecting prisms for lateral inversion, 

 and the vertical inversion followed as 

 a matter of course. 



Such was the original instrument, 

 made for me by Ross, which gave him 

 so much satisfaction. 



It nevertheless soon occurred that 

 the large dividing prisms above the 



objective were a mistake, as by using 

 smaller prisms which could be inserted 

 in the objective so as to nearly touch 

 the back combination, there was ap- 

 parently no limit to the power which 

 could be used ; at the same time, it 

 became evident that the upper prisms 

 were,- with their three surfaces, an 

 optical error when one reflecting 

 surface only was required — this led 

 to the substitution of a plate of glass, 

 silvered on one side and black (po- 

 lished) on the other, which, by rota- 

 ting on a horizontal axis, could be 

 instantly changed from its ordinary 

 use (with its silver side) to that of an 

 anilizer, for which purpose, as a still 

 further improvement, the bodies had 

 been fixed at the polarizing angle. 



The result of the substitution of 

 the smaller prisms was as anticipated, 

 and I was thus enabled, at one of our 

 " scientific evenings," to show " Po- 

 dura" under a -gVinch* with both fields 

 perfectly illuminated — whilst the re- 

 flecting plate disposed of the receiv- 

 ing and transmitting surfaces of the 

 original prisms as well as of the mass 

 of glass of which they were con- 

 structed. 



In conclusion I may add that the 

 fact that Professor Riddell had ever 

 directed his attention to the question 

 of erecting the image in any form 

 was quite unknown to me, and I will 

 only repeat that I as fully disclaim 

 now all intention of claiming priority 

 for my original instrument, as I did 

 eight years ago, when Mr. Crisp kindly 

 drew my attention to the subject, nor 

 do I regret (as far as I am concerned) 

 that colonel Woodward did not see, 

 or had forgotten, the disclaimer which 

 I then publicly made, as it affords 

 me another opportunity of doing jus- 

 tice to Professor Riddell, although, 

 as I have shown, I was not in fact in- 

 debted to him for any of the ideas 

 which led me to the invention. 



J. Ware Stephenson. 



London, January 4th, 1881. 



* Recorded in the Monthly Microscopical 

 youmal. 



