160 O. N. Rood-—Nature and Duration of 
Hence it is plain that forty billionths of a second is quite 
sufficient for the production on the retina of a strong and dis- 
tinct impression ; and as the obliteration of the micrometric lines 
in the experiment referred to, could only take place from the 
circumstance that the retina retains and combines a whole series 
of impressions, whose joint duration is forty billionths of a 
second, it follows that a much smaller interval of time will 
suffice for vision. If we limit the number of views of the lines 
presented to the eye in a single case to ten, it would result that 
four billionths of a second is sufficient for human vision, thoug 
the probability is that a far shorter time would answer as well, 
or nearly as well. All of which is not so wonderful, if we ac 
cept the doctrines of the Undulatory Theory of light; for ac- 
cording to it, in four billionths of a second, nearly two and 4 
half millions of the mean undulations of light reach and act on 
the eye. 
New York, June 30th, 1871. 
Art. XXIV.—On the nature and duration of’ the discharge of 4 
Leyden Jar connected with an Induction Coil; by OGDEN N. 
Roop, Prof. of Physics in Columbia College. 
PART SECOND. 
In the first part of this paper* I described certain results 
obtained with a Leyden jar of moderate size connected with at 
induction coil; measurements of the total duration of its dis 
charge were given, and it was shown that the luminous effects 
were mainly concentrated in the first act, which was found 80 
short as to be quite immeasurable with the means then at my 
sposa s one main object in these experiments was the 
production of an illumination as nearly instantaneous as poss! 
ble—the intention being to employ it hereafter in a totaly 
different investigation—it occurred to me that the desired end 
might be still more perfectly attained by the use of a quite 
small electrical surface. In the set of experiments above meh — 
tioned, the coating of the jar was 114-4 square inches, so this 
was now replaced by a jar with a coating of only eleven square — 
inches. The sparks it furnished, when connected with the same — 
induction coil, were perfectly satisfactory as regards illuminat 
ing power; and I at once proceeded to measure their duratio? 
employing the means and apparatus already described. For : 
the mirror I used silvered glass, the polished silver side reflect 
ae light; its size was half an inch square. The rod fi 
removed, with of course great advantage to the ¥© 
* See this Journal, vol. xlviii, Sept. 1869. 
