744 



Transactions of the Society. 



To find this necessary direction I construct a diagram, in 

 wliicli lines are drawn from the object to opposite points of the 

 mirror (those which indicate the extreme angular magnitude of 

 pencil allowed by size and distance of the mirror from the object 

 and its inclination to the axis of the Microscope), and project 

 according to the law of reflection lines outward into space which 

 indicate the exact arc of sky from which the light should come. 

 Fig. 66 shows the lines 1 and 7 including an angle of 30^. If 

 nothing intervene, the Hght of that sky surface must fall upon the 

 mirror and be reflected on 0. The intermediate rays 2, 3, 4, 5, 6, 

 have each their particular angle of incidence and reflection, and 

 form the converging illuminating pencil, or rather skeleton 

 outline of it, since the interspaces are filled by rays not indicated. 

 (N.B. The diagram shows and corrects the mistake of drawing in 

 Fig. 3, p. 518, vol. ii.). The magnitude of pencil is of course 

 primarily dependent upon the presence of adequate light surface, 

 but its extreme limit depends upon the size of the mirror and its 

 nearness to the object. 



Fig. 66. 



y 



a b, mirror : diameter = 2^ inch ; aob, angle of pencil = 30° ; o, object ; 

 SHS', arc of sky surface which delivers light on ah, subtending angle of 30°; 

 1, 2, 3, 4, 5, 6, 7, rays falling with variously oblique incidence on mirror ; v o, 

 vertical line = axis of Microscope ; H o', horizon line ; S o' is', angle of 30°. 



In the next place, it is to be observed that parallel beams inci- 

 dent in the direction of the extreme outside rays of the illuminating 

 pencil drawn in Fig. 66 and occupying the whole smface of the 



