Bluminaiion ivith Plane Mirror. By Dr. H. E. Fripp. 747 



useful in illustration, prove nothing unless they themselves are 

 proved ; or that they mislead when employed to demonstrate phe- 

 nomena in which the effects of distance which cannot be repre- 

 sented are main elements of the question ; as, for example, where 

 it is asserted that sky and cloud hght must by reason of their 

 distance fall with parallel incidence of rays. In the discussion of 

 such a doctrine it is pertinent to inquire what that distance is, and 

 what relation it bears to the extent of luminous surface which can 

 be brought into play. In regard to the parallelism of the direct 

 solar rays there is of course no question. But the parallelism of 

 that portion of solar hght which goes to form the firmament in our 

 own higher atmosphere is so completely broken up by repeated 

 refraction and reflection amongst the subtle particles of this higher 

 atmosphere, that the rays which constitute our dayhght fall from 

 every point of the visible heavens (though with greatly diminished 

 intensity). That is to say, we have at disposal a light source 

 extending over 180^, while the sun itself extends over a visual 

 angle of but half a degree ! Being thus surrounded by an illimitable 

 and self-luminous expanse of ether undulations, the question is no 

 longer of parallel rays only, but of light emanating from an outer 

 circle above the earth upon every point of the earth's surface. 

 And a mirror exposed to such a luminous atmosphere must both 

 receive and reflect from all sides, and upon all sides. If, however, 

 it be placed under the stage of a Microscope, all vertical light is 

 intercepted, and there remains nothing but the obhque incidence 

 before referred to as the starting-point of the theory of illumination 

 by converging light. But once brought to this point by the 

 consideration of general principles, we are easily carried on by 

 appeal to the law of reflection, in the demonstration of which a 

 geometric diagram stands as rightful evidence, and, as it seems to 

 me at least, affords indisputable proof that the doctrine of con- 

 verging light truly applies to the pencil by which the Microscope 

 object is illumined. 



The circumstances attending illumination by cloud-reflected 

 light differ greatly in detail, but not at all in principle. That 

 portion of solar rays which strikes upon and is reflected from the 

 cloud vapour close to the earth (in comparison with firmament or 

 sky light) retains, after reflection, nearly the same mixture of colour 

 as produces white light. But its superior ilhimiiiating power is 

 due probably to the near distance from the earth at which the 

 refraction and reflection of the solar rays begin, the reflected light 

 having but a short distance to travel. Another result of this 

 proximity is that the illumined portion of a single cloud may cover 

 a considerable arc of sky, 5' to 20' or more. And since tliis lumi- 

 nous expanse is frequently but a mile or loss from the earth, rays 

 from extreme points of the cloud must fall with obliquity of iuci- 



