INTENSITY OF VISUAL SENSATIONS 



655 



change of intensity of the stimulus, but is proportional to the intensity of the 

 stimulus already acting, Weber's law. 



This law, which is true only within certain limits, may be best under- 

 stood by an example. When the retina has been stimulated by the light of 

 one candle, the light of two candles will produce a difference in sensation 

 which can be easily and distinctly felt. If, however, the first stimulus is that 

 of an electric arc -light, the addition of the light of a candle will make no dif- 

 ference in the sensation. So, generally, for an additional stimulus to be felt, 

 it may be proportionately small if the original stimulus is small, and must 

 be greater if the original stimulus is great. The stimulus increases as the 

 numbers expressing its strength, while the sensation increases as the 

 logarithms. 



Every one is familiar with the fact that it is quite impossible to see 

 the jundus or back of another person's eye by simply looking into it. The 

 interior of the eye forms a perfectly black background to the pupil. The same 



FIG. 468. Diagram to Illustrate the Action of the Ophthalmoscope when a Plane Concave 

 Glass is Used, c, Observer's eye. The light reflected from any point, d, on retina of a, would 

 naturally be focussed at e; if the lens b is used it would be focussed at i, in other words, at back 

 of c. The image would be enlarged, as though of g, and would be inverted. (After McGregor Rob- 

 ertson.) 



remark applies to an ordinary photographic camera, and may be illustrated 

 by the difficulty we experience in seeing into a room from the street through 

 the window, unless the room be lighted from within. In the case of the 

 eye this fact is partly due to the feebleness of the light reflected from the 

 retina, most of it being absorbed by the retinal pigment. But the difficulty 

 is due more to the fact that every such ray is reflected back to the source of 

 light and cannot be seen by the unaided eye without intercepting the in- 

 cident light as well as the reflected rays from the retina. This difficulty is 

 surmounted by the use of the ophthalmoscope. 



The ophthalmoscope, brought into use by Helmholtz, consists in its simplest form 

 of a concave mirror with a hole in it. The one described is one of the less intricate of the 

 modern instruments. It consists of, a, a slightly concave mirror of metal or silvered glass 

 perforated in the center, and fixed into a handle; and 6, a biconvex lens of 6 to 8 cm. 

 focal length. Two methods of examining the eye with this instrument are in common use 

 the direct and the indirect : both methods of investigation should be employed. A nor- 

 mal eye should be examined. A drop of a solution of atropine (two grains to the ounce) 

 or of homatropine hydrobromate should be dropped into the right eye only about twenty 

 minutes before the examination is commenced; the ciliary muscle is thereby paralyzed, 



