4 Photographs on the Retina. 



colour-blind persons readily confuse red and green, but 

 rarely red and blue. An important question raised is about 

 the probability that in every act of visual perception there 

 is a picture of the object seen printed on the retina by the 

 action of light on this pigment. If this is so, we may sup- 

 pose that the nerve fibres are stimulated in varying degrees 

 by the colouring matter, according to the extent to which it 

 has undergone the bleaching process. Of course it is easy 

 to point out difficulties attaching to such opinions. It must 

 be regarded as certain, however, that in the retina we have 

 not merely a sensitive surface, like the photographer's plate, 

 but a self-acting photographic workshop, the retina not only 

 receiving an impression, but wiping ofi" the old picture and 

 charging itself in preparation for another. Speculations on 

 the subject for the present have perhaps little value, and 

 exact knowledge is likely to increase slowly, since in animals 

 we can scarcely know with certainty how much is actually 

 seen, and man cannot be made the subject of experiments. 

 Of course new modes of investigation may unexpectedly be 

 discovered, and lead to unexpected extensions of know- 

 ledge. 



Art. III. — Sir William Thomson's Electric Replenisher. 

 By F. J. PiRANi, Esq., M.A. 



[Read 11th April, 1878.] 



Art. IV. — Sortie Experiments in the Gold Bullion Assay. 

 By Alfred Mica Smith, B.Sc. 



[Read 16th May, 1878.] 



The following series of assays were undertaken at the 

 suggestion of Mr. George Foord, of the Melbourne branch of 

 the Royal Mint, and performed there some time ago. The 

 demonstrations which constitute Part I. are here offered 

 as a communication in the hope that they may be of use for 

 reference by some who may not themselves have the oppor- 

 tunity of performmg the exercises, as well as by others, 



