Trans. N. V. Ac. Sci. 114 Feb. 13, 



be obtained for any given set of conditions, and the ratio of expansion 

 at maximum efficiency, of fluid, of engine and of capital, and the ratio 

 of expansion which, with a given plant, gives most work for a dollar of 

 expense of operation, may all be determined with a degree of exactness 

 only limited by the magnitude of the errors of observation. 



" That the necessity of following the direction of improvement pointed 

 out and entered upon a century ago, by Smeaton— the protection of the 

 working fluid from losses of heat, by surrounding it with non-conduct- 

 ing surfaces — constitutes the most imperative of all demands to-day 

 upon the mechanical engineer engaged in designing steam engines." 



Prof. W. P. Trowbridge made some remarks on the paper of Prof. 

 Thurston, expressing his high appreciation and interest, and treating 

 of the importance ot the subject. 



Professor Charles F. Himes, of Carlisle, Pa., presented the follow- 

 ing paper, which in his absence was read at his request by Mr. W. Le 

 Conte Stevens : 



stereoscopic notes. 



The interesting and exhaustive papers lately written by Mr. W. 

 Le Conte Stevens have recalled corroborative experiments in a 

 similar direction, made by myself about i860, while critically examin- 

 ing the brochure ' published by Sir David Brewster, which did so 

 much in connection with the lenticular stereoscope devised by him, 

 and the rapid multiplication of photographic slides for it, to popu- 

 larize the beautiful discovery of Wheatstone. By experiments with 

 large photographs, specially prepared, without instrumental assistance, 

 I found, as Mr. Stevens has in his experience, that convergence of the 

 optic axes to a positive point was altogether unnecessary for the pro- 

 duction of stereoscopic effect, as published in 1 862 and subsequently,' and 

 therefore that the binocular criterion of distances, as so clearly enun- 

 ciated by Bishop Berkeley in his Theory of Vision, must consist in 

 movement of the optic axes successively converged on different points 

 of an object, rather than in the degree of convergence of the axes. 



At the same time, while experimenting upon the application of the 

 stereoscopic principle to the solution of some scientific and metaphysi- 

 cal problems, I was led to discard the rather complicated and un- 

 satisfactory diagram suggested by Sir David Brewster for the ex- 

 planation of the Phenomenon of the Horizontal Moon,^ as well as 

 the somewhat better one suggested by Professor Ruete,'* and to 



1 The Stereoscope, London, 1856. 



2 Am. Jour. Phot., Sept. i, 1862 ; Journal of the Franklin Institute, Phila., Vol. XCII. 

 (1871), Nos. 549, 550, 551 ; Vol. XCIII., Nos. 553, 556, 556, 558. 



3 The Stereoscope, p. 201. 



4 Das Stereoscop, Leipzig, i860, p. 86. 



