J. H. Lane on a Mode of Photographing Meteors. 438 
device for marking time is an application of the revolving glass 
prism, very similar to that described in my paper on a visual 
method of comparing time between distant stations, published 
in the January number of this Journal.* Immediately in front 
_ of the object glass of the camera, a glass prism of small angle 
and sufficient area to cover the entire aperture, is made to ro- 
tate at an accurately measured rate of say twenty-five revolu- 
tions per second. ‘he prism may be replaced by an excentric 
lens, or the object glass itself may revolve on a slightly excen- 
tric axis. The consequence will be that the image of a fixe 
star in any part of the field of view will traverse the circum- 
ference of a circle every twenty-fifth of a second, and the image 
of ashooting star will combine this motion with its motion of 
translation. If the photographic surface retain a visible impres- 
sion of the looped curve or the waved curve which will thus be 
produced, then, geglecting for the present the small effects of 
. 
optical Gistortion, the line drawn midway between the two 
of time separated by the equal intervals of one fiftieth of a see- 
ond, If the period be made too brief, the impression left by the 
ad of the meteor in one sweep of the looped or waved curve, 
might possibly be obliterated by the impression of the closely 
following parts of its train, while the head is traversing the sub- 
sequent sweeps of the curve. But thers no reason to antici- 
pate from this cause any difficulty in taining a sufficiently 
pe period to determine the law of variation of the velocity or 
Irection. 
 . In the above statement I have supposed only a single camera, 
~ but it will probably be impossible in this way to command a 
sufficient extent of the heavens. A system of many cameras 
may, however, be formed, so arranged that their several optic 
axes shall cross in'a common point tn front of the object glasses. 
The object glasses may thus approximated as closely as we 
can. desire, and the several revolving prisms, or excentric lenses, 
may have a common geared connection, and the backs of the 
cameras will be readily accessible for the renewal of plates. 
When the track of a meteor, by reason of its extent or situation, 
is obtained in parts from different cameras of such a system, it 1s 
g ometiieally impossible, on account of the spherical excess, that 
the exact interval of one fiftieth of a se between the times 
#* Vol. xxix, 43. 
