NATURAL PHILOSOPHY. 171 



of up and down, and astronomers and microscopists are not aware that they 

 see things reversed. We have also no consciousness of the absolute or even 

 relative size of the image on the retina, since an object looks no larger at the 

 distance of 1 foot than of 10, although the image is 100 times as large. The 

 idea of size is a function of judgment, of reference to touch. After referring 

 to the law of complementary colors, Dr. Hilgard stated that while the central 

 part of the retina can distinguish forms most distinctly, the eccentric portions 

 seem most susceptible to intensity of light and quality of color. This every 

 one will recognize who has noticed that a faint star may be seen by looking 

 at some point near it when it can not be seen by looking immediately at it. 

 So, gazing at a faded sunset directly, it appears faintest. But turning the 

 head sideways, or up or down, while the eye still looks at the sunset, will 

 cause an increase of light and color. The same result is obtained by pressing 

 the eyes or inclining the head sideward, so that the eyeballs become slightly 

 distorted in their endeavor to maintain a horizontal position. In these cases 

 distinctness of form is sacrificed to luminosity and coloring. The colored 

 minutiae disappear, and are followed by a fusion of colors beautifully " soft." 

 This is caused by the production of the image on the lateral portions of the 

 retina. 



OX BIXOCTJLAE, VISION. 



The following is an abstract of a paper on binocular vision, read before the 

 American Association, Providence, R. I., by Professor "W. B. Rogers: 



Prof. Rogers began with the first principles on which the stereoscope is founded. 

 If a bright bead be placed on an erect phi at one end of a board five feet long, 

 and two black beads be set up in the same way near the middle of the board 

 at such distance apart that when the face is at the other end, one black bead 

 will hide the bright one from the right eye and the other from the left ; on 

 looking for the bright bead the eye will see three black beads. The middle 

 or brightest one will be made up of the right bead, seen by the right eye, and 

 the left bead seen by the left eye ; the right hand image will be the right bead, 

 seen by the left eye ; and the other, the left bead, seen by the right eye. 

 Now arrange a diaphragm, so that each eye shall see but one bead, and there 

 will be but one image in the mind made up of two objects. This is the stereo- 

 scope. Every figure appears to be where the axes of vision intersect when 

 they seem to see it. If the right eye sees only the left object, and vice versd, 

 the image will be seen nearer than the object, and larger; if, as usual, the 

 right object be seen by the right eye, etc., the image will be more remote and 

 larger. But one thing is curious : the eye must adjust its focus to the distance 

 of the object, but in binocular vision the focus of the eye is not adapted to the 

 place where the axes cross, but to the real distance of the objects. It does 

 not always do this readily, and the effort necessary to make this adjustment 

 is often the main difficulty in the use of the stereoscope. The lines that try 

 the eye most are those that are to represent perpendicular lines at different 

 distances from the eye. If you adjust the axes of the eye to the nearer wires 

 of a bird-cage, those of the opposite side must appear double, and if you look 

 at them the nearer ones will be doubled. In binocular vision this tries and 



