PHYSIOLOGY. 



657 



phenomenon observed during the course of the 

 experiments seemed to show that the duration 

 of color-impressions depends, fundamentally, 

 like color itself, upon the character of the 

 nerves affected, rather than upon the wave- 

 length of the ray producing the impressions. 

 It has been observed that when one looks 

 at a page or a piece of plain white paper illu- 

 minated by a lamp placed at one side, if the 

 hand or some other object is held between the 

 eyes so that but half of the page can be seen 

 by each organ, that part of the object toward 

 the light will look green, and the other part 

 red. The same contrasting color - sensation 

 may be produced by the use of ordinary sun- 

 light whenever it falls directly upon the side of 

 the head : a white surface will appear green- 

 ish to the illuminated eye and pale red to the 

 other. These color-sensations depend upon the 

 passage of light through the sclerotic and cho- 

 roid coats of the eye upon which it is incident. 

 The colors are weakened by light entering the 

 pupil directly from the illuminating source, 

 while they are greatly strengthened when it is 

 concentrated upon the sclerotic coat of one 

 eye. The subject has been investigated by 

 Briicke, and the investigation has been resumed 

 and continued by Prof. Henry Sewall, of the 

 University of Michigan. Brucke first proved 

 definitely that the color-sensations in ques- 

 tion are due to light transmitted through the 

 sclerotic and choroid coats of the eye as dis- 

 tinguished from that which enters the pupil. 

 The light which has passed through the semi- 

 opaque walls of the eyeball can no longer form 

 definite images, but is nearly equally diffused 

 over the retinal surface ; and, having penetrated 

 membranes richly supplied with blood, is no 

 longer white but red or rosy in tint. This red 

 light stimulates and fatigues the "red ele- 

 ments " (or " red substance ") of the retina, so 

 that when the image of a white object is 

 thrown upon the background of the eye, it ex- 

 cites into preponderance the resting "green 

 elements" of the retina, and appears green, 

 according to the law of simultaneous contact. 

 To the eye which is not illuminated by oblique 

 light the white surface takes on a tint com- 

 plementary to the green as the result of 

 subjective contrast. The physiological effects 

 of light entering the eye by other ways 

 than the pupillary openings was the special 

 subject of Prof. Sewall's inquiries. In the eye 

 on which the incident light falls obliquely the 

 green retinal elements are aroused or made 

 more sensitive, while the red elements have 

 their excitability depressed. Hence green col- 

 ors appear brighter and more saturated when 

 looked at with that eye, while red and orange 

 tints lose in brightness and purity. In the eye 

 not illuminated by side-light, the red elements 

 are enhanced, and the green ones depressed. 

 Stars may be made to appear greenish or red 

 or orange by holding an artificial light by the 

 side of the head when looking at them; and 

 discrepancies in estimating the colors of stars 

 YOL. xxiv. 42 A 



may be partly caused by unnoticed factors of 

 artificial illumination. 



That property of vision, by which we are en- 

 abled to more accurately distinguish objects 

 when glancing sidewise at them, has been made 

 the subject of recent investigations by M. J. 

 Delbceuf. Experimenting with white light, he 

 has found that the greatest acuteness of vision 

 is situated in the yellow spot of the retina, but 

 that the spot most sensitive to differences of 

 light is a line situated in the vertical meridian 

 beginning at about 30 from the yellow spot, 

 and extending to about 60 from it. It fol- 

 lows from this that the eye is particularly sen- 

 sitive to changes in luminous intensity that 

 strike the upper part of the retina, or that 

 correspond with objects at the feet, or a little 

 on one side. It is thus that we are able so 

 easily to avoid obstacles. The peripheric parts 

 of the retina convey to us the modifications of 

 the light around us, and, if we care to notice 

 further, we can look at the objects directly and 

 take in their forms and dimensions. While 

 walking, the slightest incidental details of the 

 ground, a stone, a track, a piece of wood, or 

 a leaf, if we look straight ahead, will be more 

 distinctly depicted to us the closer we approach 

 them. In following the tracks of a carriage, 

 the parts nearest to us are more distinct ; but 

 if, instead of looking ahead, we look straight 

 down at them, they become less so. The same 

 is the case with lateral vision. When, on a 

 bright night, we fix our eyes on a particular star, 

 the others in its neighborhood come all the 

 more distinctly into view, though they be actu- 

 ally smaller than the one we are regarding. 



The relative rapidity with which the sensa- 

 tions of sight, hearing, and touch are trans- 

 mitted, has been made the subject of experi- 

 ments by M. Bloch. The precise object of his 

 studies was the time which elapses between 

 the moment when the stimulus acts physically, 

 and that at which it is perceived. The instru- 

 ments commonly used in the laboratory for 

 similar purposes were employed, namely, a 

 revolving cylinder, tambours, and the tuning- 

 fork. M. Bloch found that his investigation 

 could not be pursued by the ordinary method, 

 which consists in a voluntary response to a 

 stimulus applied to the sensory organ, for that 

 method involves too many processes, with their 

 attendant sources of error. The method he 

 adopted consisted in making use of the appar- 

 ent simultaneity of sensations of sight and 

 hearing, or of sight and touch, or of touch and 

 hearing, and determining the limits within 

 which this simultaneity appears to exist. 

 Thus, a short, single, well- defined sound was 

 made, by attaching to the drum a pin with 

 wax, which was struck by a lever of steel 

 each time the cylinder was rotated. This 

 stimulus was fixed in point of time, and in- 

 variable. At the same moment that the sound 

 was produced, or just before or just after it, 

 the finger of one hand was touched with the 

 tip of a piece of whalebone, also rotating with. 



