1901.] on Polish. 565 



capabilities of the eye ; and it seems certain that a finely ground sur- 

 face would be smooth enough to reflect without sensible diffusion the 

 longest waves, such as those found by Kubens to be nearly 100 longer 

 than the waves of red light. An experiment may be tried with radia- 

 tion from a Leslie cube containing hot water, or from a Welsbach 

 mantle (without a chimney). In the lecture the latter was employed, 

 and it fell first at au angle of about 45° upon a finely ground flat 

 glass silvered in front. By this preliminary reflection, the radiation 

 ■was purified from wave6 other than those of considerable wave-length. 

 The second reflection (also at 45°) was alternately from polished and 

 finely ground silvered surfaces of the same size, so mounted as to 

 permit the accurate substitution of the one for the other. The heat- 

 ing-power of the radiation thus twice reflected was tested with a 

 thermopile in the usual manner. Repeated comparisons proved that 

 the reflection from the ground surface was about *76 of that from 

 the polished surface, showing that the ground surface reflected the 

 waves falling upon it with comparatively little diffusion. A slight 

 rotation of any of the surfaces from their proper positions at once 

 cut off the effect. It is probable that the device of submitting radia- 

 tion to preliminary reflections from one or more merely ground 

 surfaces might be found useful in experiments upon the longest 

 waves. 



In view of these phenomena we recognise that it is something of 

 an accident that polishing processes, as distinct from grinding, are 

 needed at all ; and we may be tempted to infer that there is no 

 essential difference between the operations. This appears to have 

 been the opinion of Herschel,* whom we may regard as one of the 

 first authorities on such a subject. But, although, perhaps, no sure 

 conclusion can be demonstrated, the balance of evidence appears to 

 point in the opposite direction. It is true that the same powders 

 may be employed in both cases. In one experiment a glass surface 

 was polished with the same emery as had been used effectively a 

 little earlier in the grinding. The difference is in the character of 

 the backing. In grinding, the emery is backed by a hard surface, 

 e.g. of glass, while during the polishing the powder (mostly rouge 

 in these experiments) is imbedded in a comparatively yielding sub- 



* Enc. Met., Art. Light, p. 447, ] 830 : " The intensity and regularity of 

 reflection at the external surface of a medium is found to depend not merely on 

 the nature of the medium, but very essentially on the degree of smoothness and 

 polish of its surface. But it may reasonably be asked, how any regular reflection 

 can take place on a surface polished by art, when we recollect that the process 

 of polishing is, in fact, nothing more than grinding down large asperities into 

 smaller ones by the use of hard gritty powders, which, whatever degree of me- 

 chanical comminution we may give them, are yet vast masses, in comparison 

 with the ultimate molecules of matter, and their action can only be considered as 

 an irregular tearing up by the roots of every projection that may occur iu the 

 surface. So that, in fact, a surface artificially polished must bear somewhat of 

 the same kind of relation to the surface of a liquid, or a crystal, that a ploughed 

 field does to the most delicately polished mirror, the work of human hands." 



