rans. NO XY Ae Sez. 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 ot 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 of 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 1860, 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 1862 and subsequently,’ and 
therefore that the binocular criterion of distances, as so clearly enun- 
ciated by Bishop BERKELEY in his 7heory of Vzszon, 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 Hortzontal Moon,? as well as 
the somewhat better one suggested by Professor RUETE,* and to 
1 The Stereoscope, London, 1856. 
2 Am. Jour. Phot., Sept. 1, 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, 1860, p. 86. 
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