172 W. K. BRIDGMAN ON THE PRINCIPLES OF 



tion of the ray."* Hence, in viewing the picture, instead of placing 

 the eye in the line of the reflected ray, let it be moved to a point 

 vertical to the spot upon which the incident rays fall, when the trans- 

 verse vibrations thrown up from the angle where the rays are bent, 

 reach the eye unaccompanied by any of the direct reflections, and 

 hence the freedom from " glare" or fog. 



Now, if the preceding experiment be repeated more carefully with 

 an ordinary " graphoscope," it will be found also, that the richness 

 of colour and purity of the light will depend very greatly on the 

 angle at which the light shall be incident upon the surface. Select- 

 ing a piece of glowing scarlet, or of rich crimson, as being colours 

 most sensitive to the addition of white, let a small lamp or wax 

 candle be fixed upon a movable arm, pivotted to the centre directly 

 beneath the spot of incidence, and around which it may be moved in 

 the segment of a circle, and with the direct light properly screened 

 off, it will soon be jierceived by an experienced eye that there is one 

 particular position of the light at which the colour shows to the 

 greatest advantage. Let this angle be now measured, and it will be 

 found to be about fifty-six degrees, or the polarising angle for a 

 painted surface. On moving the light so as to enlarge the angle, 

 the colour will begin to fade for want of sufficient illumination, while, 

 on the other hand, if the light be brought nearer to the centre, it 

 will become brighter and more vivid, but then it will be at the ex- 

 pense of its purity, and appear as if seen through a silvery mist of 

 fog, owing to the direct reflections getting spread up into the field 

 of view. In the illumination of an opaque object under the micros- 

 cope, the same law necessarily holds good. For day-work it is best 

 to obtain the light from a white cloud opposite to the sun. If this 

 light be tested it will be seen to be wholly devoid of polarization ; 

 but let the analysing prism and selenite be directed either to the blue 

 sky, or in a direction across the suns rays, and the light will then 

 appear strongly polarized, and which scattered polarization affords 

 the worst possible kind of illumination for the microscope. It is 

 not the mere fact of using light already in a state of polarization 

 which has to be considered, but it is the act of polarization at the 

 time, which seems to develop some other correlative force concerned 

 in the result. It has been shown by Sir Win, Grove that a beam 

 of light produces heat, electricity, magnetism, chemical action, and 



* " Nature Series, Polarization of Light," by W. Spottiswoode, LL.D., 



F.K.S., &c. 



