179 
of Edinburgh, Session 1872-73. 
of rock on which boulder rests slopes down also N. by W, This 
rock also mica slate, hut of a variety different from boulder. 
Height of boulder above river Tay 855 feet. (See page 169.) 
XI. Boulder near S.E. end of Iona, standing upright. (See page 156.) 
XII. Borgue boulder. (See page 165.) 
XIII. Two rounded boulders lying on glaciated rock. (See page 159.) 
4. Oil the Physiological Action of Light. No. TIL By 
Janies Dewar, Esq., and John G. M'Kendrick, M.D. 
Since the date of our last communication, we have continued our 
investigations, with the following results : — 
1. The light from a beam of uncondensed moonlight, though of 
weak intensity, and almost entirely free from heat rays, is still 
sufficient to alter the electro-motive power of the nerve and retina. 
2. We have examined the phenomenon in the eyes of the follow- 
ing animals : — (1.) The common newt ( Triton aquations ). (2.) The 
gold-fish ( Cyprinus auratus ). (3.) The rockling ( Motella vulgaris). 
(4.) The stickleback ( Gasterosteus trachurus ). (5.) The common 
edible crab {Cancer pagurus). (6.) The swimming crab ( Portunus 
puher). (7.) The spider crab ( Hyas coarctatus). (8.) The hermit crab 
{Pagurus Bernliardus ), and f9.) The lobster {Homarus vulgaris). 
The general results with the eyes of these various animals were 
similar to those we have previously described. The eye of the gold- 
fish and rockling, both sluggish fishes, were found to resemble each 
other, inasmuch as the variations in the electro-motive force were 
slow, and in this respect they presented a marked contrast to those 
of the active and alert stickleback, the eye of which was very sen- 
sitive to light. 
The experiments on the eyes of Crustacea are of importance, 
because they show that the action of light on the compound eye is 
the same as on the simple eye, — namely, that it alters the amount 
of the electro-motive force of the sensitive surface. The eye of the 
lobster was found to give a deflection of about 600 galvano-metrical 
degrees (the scale being placed at a distance of about 26 inches). 
Light produced a variation in this deflection of about 60 degrees — 
that is, about 10 per cent., the largest amount of variation we have 
yet observed in any eye. It was also demonstrated that the effect 
of light diminished in intensity by distance was exactly what was 
observed in the case of the simple eye. For example, at the dis- 
