580 SUMMARY OF CUBRENT RESEARCHES RELATING TO 



the structural peculiarities of tliese parts, by their ahnost universal 

 occurrence in the Vertebrata, and by their exclusively making up the 

 mosaic layer. The pigment-cells alone are never absent in any mem- 

 bers of the series (with the possible exception of albinos), and are in- 

 dicated with more or less distinctness throughout the entire kingdom, 

 in connection vrith the function of sight. 



Professor Helmholtz adds (I. c.) a further exposition of Boll's views 

 on this subject. Boll had hoped to be able to assign one of the three 

 elementary conceptions of colour to each of the three above-named 

 anatomical elements. The outer cylinders of the rods, with their red 

 pigment, were found to be most rapidly decolorized by green light, 

 and thus probably are the recipients of perceptions of green. The 

 movement of the granules of the pigment-cells were apparently 

 affected by white and blue alike in equal measure, and hence these 

 cells probably are sensitive to the violet rays. The cones are left 

 to be assigned to the perception of red, a conclusion rendered 

 probable by the slight development which they present in those, the 

 peripheral, parts of the retina which are insensitive to that colour. 

 Against Helmholtz's objection that the yellow spot ought on this 

 hypothesis to afford no sensation of green, Boll advanced the insuffi- 

 ciency of our knowledge of the structure of this part of the retina. 

 Helmholtz adduces physical facts relating to the colours as bearing on 

 the question, viz. that in accordance with their different respective 

 relations to refractive media, the violet rays would find their point of 

 concentration in front of the red ones ; that while the small amount of 

 absorption necessary for the action of the violet would be furnished by 

 the cones, the large amount necessary for the red would hardly be 

 found in the pigment-cells ; on the other hand, the gi'eat photo- 

 chemical activity of the violet rays would probably lead to the pene- 

 tration of the red pigment of the rods by a small number of them and 

 to their consequent action on the pigment-cells and other parts lying 

 behind. 



Action of Light in the Formation of Haemoglobin.* — The im- 

 portance of light in connection with the production of chlorophyll 

 having been thoroughly established by many experiments, and this 

 green vegetable colouring matter showing a close analogy with the 

 colouring matter of the blood, Messrs. G. Tizzoni and M. Fileti have 

 endeavoured to establish by experiment the fact of a similar influence 

 of light upon haemoglobin. A summary of the results of their experi- 

 ments, which have extended over several months, shows that in a 

 rabbit twenty-three days old the amount of haemoglobin diminished 

 when the animal was kept in the dark but otherwise well cared for ; 

 the weight of the body at first increased and then decreased. In a 

 few days from the commencement of this decrease (two months after 

 the beginning of the experiment) death took place. Eabbits of the 

 same age and from the same litter, and kept under the same conditions, 

 with the exception of the admission of light, were used in comparison, 

 and these invariably manifested increase in weight of body and in 

 amount of haemoglobin. 



♦ Atti Accad. Lincei, iv. (1880) p. 108. 



