171 



On the Principles of Illumination in Connexion with 



Polarization. 



By W. K. Bridgman, L.D.S. 



{Bead April 2Stk, 1876). 

 Plate xv. 



Although it appears to be universally recognised that the illumi- 

 nation of an object under the microscope is of scarcely less importance 

 than the performance of the object-glass itself, it must be fully ad- 

 mitted that the perfection of the one has greatly outstripped that of 

 the other, leaving something more to be accomplished with the latter 

 in order to bring them both up to the same level. 



It is commonly understood that an object is rendered visible by 

 the light reflected from its surface. This, however, is a misleading 

 statement, and requires explanation. Let a lamp or candle be held 

 in front of an oil-painting, and then let the eye be so placed as to 

 receive the light reflected from its surface, when it will be found that 

 nothing whatever of the detail of the picture will be visible, but that 

 only a confused glare, accompanied by an indistinct outline of the 

 source of the light, will present itself. If, however, the eye be now 

 brought to a point midway between the light and its reflected image, 

 all the previous glare will be absent, and at the same time the mi- 

 nutiae of the drawing and colour of the picture will have become 

 beautifully clear and distinct ; thus making it seem as if it were light 

 absorbed rather than reflected, which causes the object to be seen, 

 although the fact admits of a different interpretation. 



Light is supposed to be derived from undulations or vibrations in 

 an "ether" which is believed to be ubicmitous, and to permeate, 

 more or .less, all objects in nature. These waves constituting light, 

 are propagated in all directions, and being given off laterally, tend 

 thus to effect its general diffusion. Now, it is these undulations or 

 vibrations, that are spread transversely to the line of the reflected 

 ray, which enter the eye and produce the phenomenon of vision ; or, 

 as it is expressed by Dr. Spottiswoode, " the vibratiqns producing 

 the sensation of light take place in planes perpendicular to the direc- 



