OF LUMINOUS IMPRESSIONS OF THE EYE. 37 
against a stud in the frame A, so as to be kept in gear with the wheell. A 
lever L turns an eccentric in contact, near K, with the frame carrying the wheels 
H, K ; and, by moving the lever a little to the right or left, the train of gas-meter 
wheels is instantly put in or out of connexion with the wheel I. The gas-meter 
wheels having been adjusted by the hand until their indices read zero, they are 
put in gear with I, and allowed to remain so for a minute or other definite time. 
The number of revolutions of the disc F in a second will obviously be 6000 times 
the number of revolutions of the wheel H registered by the gas-meter train in a 
minute. 
The disc F is an accurately turned plate of brass 0:1 inch in thickness, strength- 
ened by a narrow flange round its circumference, rising 0°1 inch from its plane. 
An arm of brass M O turns on the axis ¢, independently of the disc. In the disc 
there is cut an aperture at / 7, embracing 20° of the circumference ; but the effec- 
tive opening is formed by the knife edges / g of the moveable arm M O and 
h 7 of a fixed piece of brass NA. The edge f g is carefully filed to radiate from 
the centre of the disc, and N / is screwed to the disc, with its edge h 2 fitting 
accurately to f g, so that when the arm M O is moved away from the piece N h, 
the opening fg forms a portion of a sector of a circle. The piece M O has 
its surface in the same plane with the edge of the flange, and accurately fits 
its inner circumference. An arc of 40° is divided on the edge of the flange, and 
the arm M O carries a vernier reading minutes. It is obvious that the angle 
of the sector f g h 7 can be accurately ascertained by taking the difference of 
the vernier readings when the aperture fg h 7 is open and when it is closed. As 
the disc F revolves with very great velocity, it is essential, in order to avoid 
vibration, that it should be quite symmetrical about its axis. In order to effect 
this, a second aperture is made in the disc, and a piece of brass is attached to it, 
the same as the aperture and piece of brass at N g, and placed, respectively, 180° 
apart from them, but which are not shown in the figure. No light reaches the 
eye from this second aperture; for by the time the disc has made a half-revolu- 
tion, the aperture 4, which transmitted light from fh, has moved out of the line 
of vision. The second aperture, therefore, simply renders the disc symmetrical 
_without affecting the optical action of the machine. 
The apertures in the wheels D and E, through which the light, transmitted by 
the sector f g h i, reaches the eye, are represented at 4, m, and». The wheel 
E has two apertures, 4, /, 180° degrees apart; and D has four apertures, 90° apart, 
of which only two are seen in the figure. The aperture /, as in the case of the 
disc, is made simply for the purpose of restoring the symmetry of the wheel E, 
which would be destroyed if the aperture / alone existed; and it will be seen 
that if the apertures m, 4, and the sector fh, be adjusted so as to come together 
into the same straight line, and to allow light to pass through them to the eye, 
by the time the wheel E has made a half-revolution, so that the aperture / has 
VOL. XXII. PART I. K 
