CHAPTER XIII. 



VISION THE EYE THE STEREOSCOPE SPECTRUM ANALYSIS THE 



SPECTROSCOPE THE TELESCOPE AND MICROSCOPE PHOTOGRAPHY 



DISSOLVING VIEWS LUMINOUS PAINT. 



THE eye and vision are such important subjects to all of us that we may 

 be excused for saying something more concerning phenomena connected with 

 them, and the instruments we use for assisting them. We do not propose 

 to write a treatise upon the physiology of vision, for we know the image in 

 the eye is produced physically in the same manner as the image in a camera 

 obscura. In the eye the sides of the box are represented by the sclerotic 

 (see chap. x. fig. 95) ; the dark inner surface has its parallel in the pigment 

 of the choroid ; the opening in the box in the pupil of the eye ; the convex 

 lens in the crystalline and the cornea ; and the retina receives the image. 

 But why we see beyond the fact that we do see no one can explain. 

 Science is dumb on the subject. Thought and consciousness elude our 

 grasp ; and, as Professor Tyndall says on this subject, " we stand face to 

 face with the incomprehensible." 



But there are many interesting facts connected with our vision which may 

 be plainly described. Some people are obliged to carry an object (or a book) 

 to within ten inches of the eye to see it distinctly; and a person who does not 

 possess convergent power of the eye will have to move it farther off, or use 

 convex glasses ; while a " near-sighted " person, whose eyes are too quickly 

 convergent, will use concave glasses to bring the object near to the eye. 



There is but one small place in the retina of the eye which admits ol 

 perfect vision. This, the most sensitive portion, is called the " yellow spot," 

 and vision becomes more and more indistinct from this point towards the 

 circumference. This can he proved by any one ; for in reading we are 

 obliged to carry our eyes from word to word, and backwards and forwards 

 along the lines of print. Another very important element in our vision is the 

 contraction and enlargement of the iris around the pupil. In cases where 

 strong light would only dazzle us the iris expands, and the pupil is contracted 

 to a sufficient size to accommodate our vision. At night, or in a darkened 

 room, the pupil is enlarged. This change will account for our not being im- 

 mediately able to see objects when we have passed from darkness to great light, 

 or vice versa. The iris must have time to accommodate itself to the light. 



Now, outside the small space of perfect vision above mentioned, there is 

 acircle of considerable extent, called the "field of vision." In man this field, 



