120 Mr. B, Brudenell Carter [May 9, 



impervious to any colour but their own. This lantern slide [s/^ozcwj 

 is taken from a drawing by Mr. Hulke, in a paper communicated 

 by him to the Royal Society, and it exhibits the colour globules in 

 the retinal cones of Chelonia Mydas. Each globule is so placed 

 as to intervene between what is regarded as the collecting portion 

 of the cone and what is regarded as its perceptive portion, in such a 

 way that the latter can only receive colour which is capable of pass- 

 ing through the globule. The retinae of many birds, especially of 

 the finch, the pigeon, and the domestic fowl, have been carefully 

 examined by Dr. Waelchli, who finds that near the centre green is 

 the predominant colour of the cones, w^liile among the green cones red 

 and orange ones are somewhat sj)aringly interspersed, and are nearly 

 alw^ays arranged alternately, a red cone between two orange ones and 

 vice versa. In a surrounding portion, called by Dr. Waelchli the red 

 zone, the red and orange cones are arranged in chains, and are larger 

 and more numerous than near the yellow spot ; the green ones are of 

 smaller size, and fill up the interspaces. Near the periphery the cones 

 are scattered, the three colours about equally numerous and of equal 

 size, while a few colourless cones are also seen [shown]. Dr. Waelchli 

 examined the optical properties of the coloured cones by means of the 

 micro-spectroscope, and found, as the colours would lead us to suppose, 

 that they transmitted only the corresponding portions of the spectrum ; 

 and it would almost seem, excepting for the few colourless cones at 

 the peripheral jDart of the retina, that the birds examined must have 

 been unable to see blue, the whole of which would be absorbed by 

 their colour globules. It would be necessary to be thoroughly 

 acquainted with their food in order to understand any advantage 

 which the birds in question may derive from the predominance of 

 green, red, and orange globules over others; but it is impossible 

 to consider the structure thus described without coming to the 

 conclusion that the birds in which it exists must have a very acute 

 sense of the colours corresponding to the globules with which they 

 are so abundantly provided, and that this colour-sense, instead of 

 being localised in the centre, as in the human eye, must be diffused 

 over a very large portion of the retina. Dr. Waelchli points out 

 that the coloration of the yellow spot in man must, to a certain 

 extent, exclude blue from the central and most sensitive portion of 

 his retina. 



It is hardly necessary to mention how completely the high 

 differentiation of the cones in the creatures referred to tends to 

 support the hypothesis of Young, that a similar differentiation, 

 although not equally m.anifest, exists also in man. If this be so, 

 we must conclude that the region of the yellow spot contains cones, 

 some of which are capable of being called into activity by red, others 

 by green, and others by violet ; that a surrounding annulus con- 

 tains no cones sensitive to green, but such as are sensitive to red 

 or to violet only ; and that, beyond and around this latter region, 

 Buch cones as may exist are not sensitive to any colour, but, like 



