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XLII.— ON PHOTOMETEES MADE OF SOLID PAEAFFIN, OE 

 OTHEE TEANSLUCENT SUBSTANCE. By J. JOLY, 

 B.E., Assistant to tlie Professor of Civil Engineering, 

 University of Dublin. 



[Eead, December 15, 1884.] 



When ligM enters a translucent body, sucli as paraffin, milk- glass, 

 common opal, &c., it behaves as if scattered within the substance 

 by numerous reflections — as if such bodies were not homogeneous, 

 but consisted of particles differing in physical properties, in so far 

 that some of them are opaque, others transparent, or as if some 

 jDartial molecular symmetry led to the existence of planes of supe- 

 rior density within the substance. Such a substance as milk-glass 

 would be an example of the first class of bodies ; paraffin possibly 

 of the second, or partly of both. 



If now a prism be cut from a translucent body, and so exposed 

 to a source of light that one only of its faces be illuminated, the 

 light diffused through the substance and reflected out through the 

 unilluminated faces of the prism gives it an appearance as if lighted 

 up internally. The effect, in fact, is as if the prism itself was a 

 source of light. Two such prisms laid together on smooth faces, 

 and receiving light from separate sources (placed so as to be at 

 opposite sides of the plane of division), appear as if each was 

 emitting light proportional in intensity to the source of its supply. 

 The double prism has the appearance of two luminous bodies laid 

 side by side. When, however, the supply to each prism is brought 

 to equality, they appear as if emitting equal quantities of Kght. 

 In fact, it is hard to detect any longer that two prisms are being 

 observed, so completely does all trace of the plane of division dis- 

 appear. 



The photometer I wish to bring to your notice consists of two 

 prisms of paraffin, parallelepipeds in form, each about 2|^" x 1'' x -i 

 sharply cut to the same dimensions, and laid together in their 2|' 

 X 1" paces. In this position they are retained by two brass rings, 

 such as are i. ^d for mounting objects for the microscope, melted 

 on at each end. 



The two sources of light to be compared are arranged at oppo- 



SCIEN. PllOC. ll.D.S. — VOL. IV. PT. VII. 2 H 



2 3 

 Iff 



