83 report— 1877. 



On the Lower Limit of the Prismatic Spectrum. By Lord Bayleigb. 



On a Binocular Microscope for High Powers. By J. Tbaill Taylor. 



Some neiv Optical Illusions. By Shvantjs P. Thompson, B.Sc, B.A. 



The relative motions of an object and a spectator may give rise to illusions in 

 which motions differing- in direction or character may he apparently produced. 

 Some illusions of this class were brought forward by the late Sir D. Brewster in 

 the 'Transactions ' of the Brit. Assoc, for 1845, 1848, and 1861. The author has 

 observed several other cases of illusory motion apparently due to a retinal compen- 

 sation of the disturbance produced by the motion of images across the retina. 

 Several of the illusions cited were observed in railway travelling. Thus, if from 

 a rapid railway train objects from which the train is receding be watched, they 

 seem to shrink as they are left behind, their images contracting and moving from 

 the edges of the retina towards its centre. If after watching this motion for some 

 time the gaze be transferred to an object at a constant distance from the eye, it 

 seems to be actually expanding and approaching. 



The author also drew attention to the following illusion: — A series of concentric 

 circles of black and white are drawn upon a card. This is held firmly between the 

 the thumb and finger, while a circular shaking motion is imparted to' it by a move- 

 ment of wrist and elbow, the circles appearing to rotate upon the card. It was held 

 that the various effects of "compensation" to the movement of retinal images 

 could be explained by supposing them to set up a secondary wave of nervous dis- 

 turbance propagated in an opposite direction to that of the primary movement of 

 the images. 



On the Relative Apparent Brightness of Objects in Binocular and Monocular 

 Vision. By Silvanus P. Thompson, B.Sc, B.A. 



It is a common idea that objects appear brighter when seen with the two eyes 

 than with one. There appear, however, to be exceptions to this statement. The 

 following is a method of submitting the question to photometric measurement. The 

 comparison-photometer employed consists of a cardboard screen having an aperture 

 divided into two equal portions. One half is covered with tissue-paper, and illu • 

 ruinated directly from behind. Behind the other half is set, at the polarizing angle, 

 a mirror of black glass. Light from a second lamp falls upon a screen of tissue- 

 paper, whose light is then reflected in the mirror. Thus the two halves of the aper- 

 ture may be illuminated equally, but with light in one case wholly unpolarized, in 

 the other wholly polarized. Let two Nicol prisms be now taken, having their prin- 

 cipal sections placed parallel and perpendicular respectively to the plane of polari- 

 zation of the mirror, and let one Nicol be placed in front of each eye. One eye only 

 will receive the whole of the polarized light, while the unpolarized light will be 

 equally distributed, half to each eye. The total amount of light received upon the 

 retinal surface will be the same from each half of the aperture ; but their apparent 

 illuminations will be unequal, that of the polarized light appearing the greater. By 

 comparing the distances at which the lamps must be placed it appears that light is 

 more powerful in producing an effect when concentrated upon one eye than when 

 equally distributed to the two, though according to what law experiments are not 

 yet sufficiently numerous or exact to determine ; but, on the other hand, the light 

 so concentrated on one eye does not produce the sensation of twice as much illumi- 

 nation as the half of the light viewed by both eyes at once. 



