SCHULTZE, ON THE RETINA. 33 
perception, propounded by Young and Helmholtz, that at 
least three different kinds of fibre must be required for this 
perception. Each “cone,” therefore, in the mammalia and 
fishes, having this compound structure and all being alike, 
it would appear to follow that all are equally capable of per- 
ceiving every variety of colour. And his argument is still 
further strengthened by the consideration that, inasmuch as 
all or nearly all the “ cones” in a bird’s retina are furnished 
with a coloured spherule, through which all the light reach- 
ing the percipient part must pass, it would be absurd to 
suppose that they were incapable of receiving impressions of 
colour, for which, so far as shown by that circumstance, they 
alone would seem to be fitted. Furthermore, it is to be borne 
in mind that all the “ cones” in a bird’s eye do not contain 
spherules of the same colour, and that some are without any, 
whence we may conclude that in all probability the differently 
coloured “ cones’ are adapted for the perception of mono- 
chromatic light corresponding to that of the spherule con- 
tained in them, and that each is not, as in the mammal, 
capable of conveying equally impressions of all colours. And 
this view is curiously in accordance with the circumstance 
that the “cone ”-filaments in the bird are scarcely thicker 
than those of the “rods.” Whether this be the case with 
the filaments proceeding from the colourless * cones,” has not 
been made out. But it may be that these “cones” are adapted 
for the reception only of the violet rays, which would, of 
course, be absorbed in their passage through the coloured 
* cones.” 
‘The structure of the owl’s retina, in contrast with that of 
diurnal birds, may be cited in support of the same argument. 
And the author refers to a suggestion of his own, made in a 
former paper on the macula lutea,* that the intervention of 
the yellow spherule in birds, and of the yellow colour in the 
human macula, may serve for the interruption of the more 
powerful photo-chemical rays in their passage to the delicate 
percipient tissue. 
This part of the paper concludes with a highly interesting 
disquisition respecting several other points connected with 
the simple visual sense and the estimation of sizes and forms, 
&c., for which the reader must consult the original. 
* ‘Ueber den gelben Fleck der Retina,’ &c. Bonn, 1866. 
+ Should M. Schultze’s ingenious speculation respecting the use of the 
yellow and red spherules in the retina of birds and some sun-loving reptiles 
be entertained, it would seem to suggest the propriety of using yellow 
glasses to protect the eyes in strong daylight, as on snow or at sea in the 
tropics, for instance, instead of blue or violet ones, which transmit only the 
very rays which nature seems to be so solicitous to intercept. 
VOL. VII.—NEW SER. Cc 
