140 



Profs. W. E. Ayrton and John Perry. [Dec. 12. 



This condition, however, of proximity of the screen to the mirror 

 ceases to have any weight, and the phenomenon can be shown to a 

 large audience by projecting it on a distant wall if one or other of the 

 following devices be adopted : — 



1. Allow the sunlight to first pass through a small hole, so that all 

 the rays falling on the same point of the surface of the mirror make 

 the same angle with the surface. 



2. Obtain the same result thus : — first let the sunlight fall on a 

 convex lens or on a concave mirror which brings it to a focus, and 

 afterwards causes it to diverge from a single point, then hold the 

 Japanese mirror in the diverging beam at about eight or more feet 

 from the principal focus of the lens or auxiliary concave mirror. 



3. Illuminate the mirror with light diverging from a single bright 

 point at some distance, as, for example, from an electric light at the 

 other end of the room, the screen, of course, being shaded from the 

 direct light of the lamp. 



Fig. 4. 



4. Allow the sun's rays to fall on the nearly plane Japanese mirror 

 MM, and after reiiection let them pass through a converging lens so- 

 adjusted that the screen KR, fig. 4, is beyond the principal focus, P, of 

 the mirror and lens combined, and also beyond JJ, the conjugate focus 

 of the mirror, that is the place at which the image of the Japanese 

 mirror is formed by the lens. 



The last method causes the effect to be better than that obtained 

 with ordinary sunlight alone, because the insertion of the lens 

 separates the rays falling on different points of the Japanese mirror 

 more than it separates those which, coming from different points of 

 the sun, are reflected in different directions by the same small portion 

 of the Japanese mirror. In fact, the employment of the lens corrects, 

 to a certain extent, the blurring of the image produced by the sun 

 not being a single luminous point. Number 4 method also economises 



