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Transactions of the Society. 



increasing its angular convergence on the object. Any simple 

 collecting lens will perform the same service as the most compli- 

 cated lens combination if it have suitable focussing arrangements, 

 and its refracting surfaces are so controlled by diaphragm apertures 



Fig. 9. 



Fig. 9, to show action of plane mirror and collecting lens 'without diaphragm 

 aperture. 

 P /, Focal plane of object. 

 C L, Collecting lens. 



P L, Illumiuatiug cone emitted from C L, whose light is derived from 

 the surface of mirror A B. 

 Its angle of convergence aud specific intensity = that which the larger mirror 

 surface a b reflects without intervention of lens, and supposing the light-source to 

 have sufficient extent to give the incident cone c a h d. 



N.B. It tlius appears that, 1, where tlie angular convergence of illuminating 

 cone depends on plane mirror reflections, the size of mirror and requisite exten- 

 sion of light-source set limits to its use ; 2, that an artificial light-source of 

 small extent, tlie reflection of which from mirror gives a diverging cone, is (within 

 certain limits) more conveniently converged on the object by a collecting lena 

 properly arranged. 



as to exclude colour ; a concave mirror being employed when the 

 light-source is of limited extent. The magnitude of the incident 

 light-cone will be in proportion to the proximity of the diaphragm 

 to the object. Its focal point need not fall exactly on the object, 

 though the maximum of hght is attained when it is so focussed. 

 But a diaphragm aperture, which, being too large or too near the 

 object, allows an excess of light to fall on it, is disadvantageous ; as 

 also when the aperture bounds a light-cone to which the mirror 

 offers an insufficient base. 



To satisfy every condition the apparatus should in addition to 

 the diaphragm over the last refracting surface * which regulates 



* See the Handbook of Professors ■Niigcli and Scliwendener, p. 97, para- 

 graph 87. 



