778 OP THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



the pressure of the retina ; and as it seems to our perceptive consciousness to 

 have a distinct objective existence, and as its position bears a constant and 

 definite relation to that of the portion of the retina on which the impression 

 is made, it seems obvious that any such affection of the retina not only im- 

 mediately suggests to our minds the notion of an external objective cause of 

 the impression, but also indicates to our consciousness the direction of the 

 object. But further, besides the principal " phosphene," another, of smaller 

 dimensions, is usually to be seen, in a direction nearly the same as that on 

 which the pressure is made ; this is the result of the transmission of the 

 pressure to the opposite side of the globe, by an alteration of its figure and 

 of the position of its contents, which corresponds to the fracture of the skull 

 by contre-coup. The form of this smaller or secondary " phospheue " is not 

 affected by the cause which sometimes renders the larger or primary spec- 

 trum incomplete ; since, as we cannot anywhere apply pressure to the living 

 Human eye, save on some part of its anterior hemisphere, the " coutre-coup " 

 will always take place at the opposite spot in the posterior hemisphere, over 

 which the retina is continuous, save at the entrance of the optic nerve. By 

 an extensive series of observations upon the relation of the positions of the 

 primary and secondary " phospheues," both to each other and to the seat of 

 compression, Dr. Serre has deduced the important conclusion, that the lines 

 joining the imaginary spectra and the spots of the retina upon whose affec- 

 tion they respectively proceed, pass through a common "centre of direction," 

 whose position is in the middle of the crystalline lens. And hence it seems 

 to be a legitimate conclusion, that our sense of the relative directions of ex- 

 ternal objects is derived from a kind of mental projection of each point of 

 the retinal image, in the line which joins it to this " centre of direction." 



636. Another very curious subjective phenomenon of Vision, is the repre- 

 sentation which, under particular circumstances, we may mentally obtain of 

 the retina itself; as in the following experiment, first devised by Purkinje, 

 and known by his name. "If in a room otherwise dark, a lighted candle be 

 moved to and fro, or in a circle, at a distance of six inches before the eyes, 

 we perceive, after a short time, a dark arborescent figure ramifying over the 

 whole field of vision ; this appearance is produced by the vasa ceutralia 

 distributed over the retina, or by the parts of the retina covered by those 

 vessels. There are, properly speaking, two arborescent figures, the trunks of 

 which are not coincident, but on the contrary arise in the right and left 

 divisions of the field, and immediately take opposite directions. One trunk 

 belongs to each eye, but their branches intersect each other in the common 

 field of vision. The explanation of this phenomenon is as follows : By the 

 movement of the candle to and fro, the light is made to act on the whole 

 extent of the retina, and all the parts of the membrane which are not im- 

 mediately covered by the vasa ceutralia are feebly illuminated ; those parts, 

 on the contrary, which are covered with those vessels, cannot be acted on 

 by the light, 'and are perceived, therefore, as dark arborescent figures. 

 These figures appear to lie before the eye, and to be suspended in the field 

 of vision ;" * and as the vessels appear magnified, and display a remarkable 

 purallactic gliding movement over the visual field, data are afforded by 

 which II. Muller has calculated that the true percipient surface of the retina 

 must be situated at a certain distance from the vessels, corresponding in fact 

 with the bacillar layer. 2 We have thus another demonstration of the fact, 



1 Muller's Elements of Physiology (Baly's Translation), p. 1163. 



2 For the explanation of various other cntoptical phenomena, see James Ja^o, 

 H.A., in rroiTedinir* ,,f the Royal Society, vol. viii, p. f>03. He observes that the 

 long brains of light which issue from flames regarded with "winking eyes" proceed 

 from little bars of fluid along the margins of the lids. 



