of Edinburgh, Session 187 8-7 9. 
39 
Sect. IV. The Second or Transmission Method. — Prick a pin-hole 
in tinfoil. Shut one eye and bring the hole within 12 mm. off the 
open eye. 
Expts. 1, 2, and 3 may readily he repeated by this method. 
Expt. 4. Place green glass before the aperture, and notice the 
size of the field, then withdraw the glass suddenly. The pupil 
contracts. Red glass gave the same result. 
Sect. Y. The Third or Refraction Method. — The following ex- 
ample will make perfectly clear the way of working here : — I am 
looking at a star, with the moon at full, a little to one side. Prom 
the star proceed the rays mentioned at the close of Sect. II. Upon 
turning towards the moon, but still keeping my attention con- 
centrated on the star, the rays of the latter appear to retreat into it ; 
and upon turning from the light of the moon, the rays emanate 
from the star again. 
Expt . 5. This is typical of about seventy other experiments I 
have made. The night is starless. An isolated gas lamp, with no 
houses near or any other sources of light, appears when seen from a 
distance with the usual rays emanating from it. I walk towards it 
slowly. At 300 yards no alteration has taken place in the rays; 
they appear fixed. The distance is slowly decreased, hut not until 
I am at a distance of 16 yards do the rays perceptibly shorten; in 
other words, the light from this one gas lamp is incompetent to 
effect a movement of my iris until I am within 16 yards of it. The 
shortening of the rays is now rapid, for at 10 yards distance the 
light appears to he without them. 
Expt. 6. In the preceding experiment there is a possibility that 
the rays may he shortened to some extent by the increase in size of 
the image on the surface of the cornea as we near the light. In the 
present experiment this objection is to some extent removed. Two 
gas lamps were chosen, 50 yards apart, and whilst walking towards 
the nearest my attention was kept exclusively on the rays emanating 
from the farthest one. As the first lamp is approached, the effect 
of its light on the iris is visible in the alteration of length of rays 
proceeding from the far one. Thus in the fig. the two lamps are 
A and B, and the observer stationed at 0 sees rays emanating from 
both. A is the lamp whose influence on the iris is to be tested, 
and B is the lamplight used as a tester. Proceeding from O 
