746 



Transactions of the Society. 



remaining parallel after reflection, fail to strike the object. In 

 fact, while those rays which do fall with such incidence as to be 

 reflected on the object, form a converging pencil of given angular 

 magnitude (e. g. 30^ in Fig. 66), the several bundles of jparaUel 

 rays falling from the same area of light source would after reflection 

 occupy a space at the plane of the object (see dotted hues in 

 Fig. 67) half as large again as the mirror ; scarcely a suitable illu- 

 mination for a microscopic object ! 



Lastly, the illuminating power of diverging rays, supposing 

 them to proceed from a single point of light source and spread 



Fig. 69. 



Fig. 70. 



V axis of Microscope, the instrument being reclined to get the whole pencil 

 above 'line of horizon ; ab, mirror inclined at 45° ; aob, angle of illuminating 

 pencil = 30°, subtended by S S', arc of sky ; A, axis of illuminating pencil 

 reflected on o, object ; S b and S' «, outlines of cone reflected on object o 



The dotted lines in both diagrams show that no other rays but S 6 and b a 

 touch the object, as the other diverging rays from the points b and b taUing witn 

 greater or less obliquity on the mirror surface, are reflected in directions more 

 and more remote from the object. 



over the mirror smiace, as in Figs. 69 and 70, is too slight and 

 moreover too scattered to add anything to the total efi'ect. 



But it may be (perhaps fairly) objected that diagrams, though 



