Transit Observation without personal error. 57 
position in the field for a moment only, though the use of a 
ash of light which for an instant illuminates the wires has 
seemed better. In any case the essential idea was to treat the © 
eye as it has been proposed to use the photographie plate, and 
to impress upon it by instantaneous vision an image of the 
position of the star independent of personal error (in one sense,) 
since the star was to have sensibly no time to move, while the 
view lasted, while yet the effect then seen would remain (owing to 
persistence of vision,) quite long enough for cognizance. After 
a great number of forms of the movable screen were considered, 
they were set aside for the flash of light from the electric spark. 
To make the present plan plain, let us recur to the use of 
photography as an illustration, and first suppose that the transit . 
wires are twenty-five in number and so near that a star occupies 
one second in passing from one to another. In the place of a 
transit lamp, let a wire, attached to a repeater in the clock 
circuit, dip in a mereury cup, and the circuit being broken 
every second by the clock, recording simultaneously on the 
chronograph; the otherwise dark field will be illuminated by a 
brilliant instantaneous flash, by whose light the wires are 
projected with the crossing star on the sensitive plate of the 
camera. If this plate be renewed or moved downward at each 
flash, we obtain twenty-four successive pictures while the star 
1S Crossing, each showing it at the extremity of its luminous 
traces in precisely the same relation to the adjacent wires, since 
the wire interval in time, and the flash interval, are identical. 
If then the star is caught by the first flash, one-third of the 
those we could expect from the photograph, and equally exempt 
rom personal error, in the sense in which we have defined it. 
tst as to the difficulties to overcome. — In the case of the 
photograph, there is an opportunity for subsequently measuring 
the distance of tie star from the wire, which we have not with 
© eye. There is one particular case however, where the 
result is the same for both, that, namely, when the flash comes, 
the Star is on the wire and bisected by it; in this case we know 
US position as accurately by the eye, as if we bisected its image 
‘on the plate by the wire of our measuring micrometer, with — 
