46 CORBESPONDENCE. 



at all. He iinderstood, however, tliat it was Mr. ToUes' eye-piece ; 

 and I have by me the original " exhibitor's card " at one of the soirees 

 named, reading distinctly, " Tolles' binocular eye-piece, exhibited by 

 Prof. Smith." I feel very anxions to have the mistakes corrected. 



The fii'st eye-piece of this form which Mr. Tolles ever made was 

 purchased by me, and I gave some account of its performance and 

 peculiarities in the 'American Joiu'nal of Science,' July 1864, p. 111. 

 And, in the same journal shortly after, Mr. Tolles himself described 

 its construction. Now, although we cannot expect everyone to read 

 an American journal, or to be posted in all that is done this side of 

 the Atlantic, even in the microscoi)ical line (excej)t to notice the 

 trash, e. g. the mean little sheet issued by manufacturers and sellers 

 of the Craig ! ! ! microscope, recently attempted to be palmed off as the 

 organ of the Illinois Microscopical Society), it is a little siu'prising, 

 considering the length of time these two articles have been published 

 in a most prominent journal, that such a blunder shoiild occur. Of 

 course it is inadvertence. Dr. Carpenter, I am quite sure, is ready 

 and willing to do full justice, and in so doing it will be proper to 

 state the real principle of the eye-piece in question. It is not, as he has 

 stated, I. c, merely an arrangement of prisms similar to MM. Nachet's, 

 for in reality the prime part of the eye-piece may be this, or Riddell's, 

 or Wenham's ; and, in fact, in the first eye-piece made for me was 

 different from either of these. Mr. Tolles finally — partly at my sug- 

 gestion, though I believe he had already decided upon it — adopted 

 the Nachet form, and he claims nothing for this. What he does 

 claim, and is justly entitled to claim, is the construction of a first- 

 class achromatic erecting eye-piece, and a division of the j^encil, for 

 stereoscopic vision, at, or very near, the point of crossing of the rays 

 in such a combination. Now, it is well known that the difficulty in 

 using the Wenham, or Nachet arrangement with high powers, arises 

 from the necessity of dividing the pencil so far behind the objective, 

 a difficulty which it seems cannot be got over, except upon Mr. 

 Tolles' plan, viz. making a secondary image, and dividing the pencil 

 here, or near the point of crossing of the rays. The binocular eye- 

 pieces invented by President Barnard, and by myself, are simply bino- 

 cular, like Powell and Lcaland's arrangement for high powers, though 

 superior as to equality in illumination of the two fields, they are not 

 stereoscopic. Perhaps the fact of my having made such an eye-jiiece, 

 and published an account of it, as also Dr. Barnard's notice of it, in 

 his reijort upon the Paris Exposition, may have assisted to mislead in 

 attributing the really stereoscopic binocular of Mr. Tolles to me. If 

 I had been the originator of tliis eye-piece, which is yet destined to 

 replace most binoculars, I should feel I had contributed a much 

 greater boon to microscopy than anything I have yet done. The 

 instrument, as made now by Mr. Tolles, is very perfect ; the loss of 

 light is trifling, easily remedied by a little more illumination. The 

 loss in definition is not so much as in the Wenham and Nachet forms ; 

 not merely from the care with which Mr. Tolles works the prisms, 

 but owing to the much shorter distance which the reflected ray has to 

 travel. This part of Dr. Carpenter's objection is practically without 



