312 W. K. BRIDGMAN ON THE 



Now, if the illumination of the ohjeet and the seeing it with the 

 object-glass and eyepiece were separate and independent actions, 

 such an effect would not be produced ; but the object would remain 

 stationary under both conditions. The true state of the case is that 

 the illuminating rays, impinging upon the object, pass through the 

 body of the instrument in one continuous or unbroken line, project- 

 ing forward the image of the object until it reaches the eye ; hence, 

 whatever degree of obliquity is given to the illuminating rays, it 

 must of necessity affect the results given by the object-glass, and 

 thus, whether the plane or the concave mirror be used, the effects 

 will differ in proportion. 



In the experiment with the painting seen in the graphoscope, it 

 was shown that when the illuminating ray, reflected from the surface, 

 entered the eye, nothing could be seen but the glare of the light 

 itself, the detail of the picture being totally invisible. This same 

 law, necessarily, applies equally to the direct ray reflected by the 

 mirror up through the centre of the optical system. The central 

 portion of every lens, presenting more or less surface at right angles 

 to the course of the ray, suffers this to pass without refraction, or 

 bending out of its course, and hence it carries with it only the glare 

 of the light as it did in the graphoscope. This effect, however, 

 decreases of course in proportion as the zonal ring, admitting the 

 rays, recedes from the centre, the refraction being greatest at the 

 exterior margin. 



It was also explained that it is not the direct or reflected ray 

 which the eye takes cognizance of as rendering an object visible, but 

 that it is the transverse vibrations given off laterally which 

 enter the eye and produce the sensation of vision. Now, in 

 accordance w'th this law, rays of light may be reflected or refracted 

 from objects at too small an angle for any of these lateral vibra- 

 tions to reach the eye, and consequently the objects will be invisible. 

 The extent of this angle is a disputed point, but it is supposed to 

 be about equal to a half-wave length ; but what more immediately 

 concerns us in the present case is that in proportion as the angle 

 approaches to 180°, or to a straight line, the more of these lateral 

 vibrations enter the eye, giving the greatest possible distinctness to 

 the object : hence in this lies the chief reason of wide-angled object 

 glasses having more light and giving greater distinctness of vision, 

 although it must necessarily be at the expense of " deptli of focus " 

 or penetration. 



