TRANSACTIONS OF THE SECTIONS. 183 
Another exception may be taken to the weight which all these levers will add to 
the mounting. As however it will not be difficult to make the proportions of the 
arms as ten to one, the additional weight will scarcely exceed a tenth of the weight 
of the speculum. 
The apparatus will evidently only act perfectly, when the bars (i. e. lines running 
along the axes of the bars) at the back of the speculum are in a horizontal position ; 
but I presume that such a variation from that position as might be produced in a 
couple of hours’ observation by carrying on the tube, could not produce any sensible 
effect. 
On the Powers of Minute Vision. Results from Experiments for determining 
the best sort of Station-marks, and the errors liable in observing with Op- 
tical Instruments that measure on the principle of bringing two reflexions 
together. By ΝΥ ΑΜ PETRIE. 
The experiments were performed in bright daylight (but not sunshine), being light 
of the maximum of advantage for perceiving black against a white ground. The 
general circumstances of the experiments were arranged rather to determine the facts 
of common practice, than the theoretic powers of vision. 
The author then detailed the various distances at which circular spots, lines, &c., 
white on black, as well as black on white, could be seen, the distances being given 
in terms of the breadth of.the object seen. An arrangement of lines was described, 
by which an alteration of their position to the extent of only one-millionth part of 
the distance of the observer was made visible. 
One result of the experiments would be to show what should be the proper pro- 
portions of parts to be observed in forming letters to be read with the greatest di- 
stinctness at a distance, a subject of much practical use in the present day, and 
admitting of a strictly scientific system, although generally left to the fancy of in- 
competent persons. White letters on a black ground should have their component 
lines of only half the breadth that black letters should have on a white ground. 
The direction of the eye, while appearing to gaze steadily at any object, does in 
reality keep wandering to an imperceptible distance on every side of the object looked 
at, but very rapidly. This wandering is not accidental, or an imperfection of sight, 
but an essential feature of vision ; because it is not the continuance of an impression 
that is perceived (by any of the animal nerves), but its commencement and termina- 
tion, or, more strictly speaking, its increase and decrease. This principle is pro- 
bably analogous to that by which a magnet creates an electric current in a neigh- 
bouring wire, not by its constant presence, but by the increase or diminution of its 
influence, either by a variation of its power or of its position. This wandering pro- 
pensity of the eyé was shown to account for the relative facility with which different 
sorts of marks were seen at great distances: it takes place, apparently, in a mini- 
mum case, to the extent of an angle of one in 2500. 
A dislocated line (as in a vernier), its falt being half its breadth, can be perceived 
to be so at a distance of 10,000 times its falt (if black on a white ground), and at 
12,000 times if white on a black ground. It shows itself, however, by giving the 
line a less steady appearance (than a perfectly even line would have) when narrowly 
watched by running the eye along the line, at about half as far again. 
Experiments were then described on the visibility of the positions of the ends of 
lines, and of hiatuses in lines, and of square dots as compared with round. 
But the last conclusion of practical importance was, in respect of observing the 
angular position of station-marks, or of stars, by reflexion as in a sextant. From 
these experiments it appeared that the position of two closely adjacent dots or 
‘images, in sensible parallelism to a given direction, while it affords one of the sim- 
plest kinds of observation, is more accurately observable than their actual coinci- 
dence, or even than the junction of two lines as if in a vernier. 
On the Application of Electricity and Heat as Moving Powers. 
~ By Wiuu1aM PETRIE. 
From the dynamic equivalent of electricity already given we can infer an import- 
