514 Transactions of the Society. 



large aperture, rvhen the object contains such detail, the greater 

 admission of light through such an objective when viewing an 

 object which does not contain such detail, severely tests its capacity 

 of definition by flooding out the delineating shadows. Thus the 

 benefit of a wide illuminating cone is confined to particular cases, 

 and is not of general application. 



Further, as respects the necessity of a diflferent condenser for 

 every object : everyone is of course aware in our day that a single 

 illuminating lens, if constructed to admit the widest pencils re- 

 quired for any objective, can also be used for objectives of lesser 

 aperture, when the incident Hght is regulated by a diaphragm 

 properly placed at the plane of its lower focus, and supplied with 

 openings of the required size. But this regulation of the size of 

 incident pencils did not enter into the design of the Brewster 

 illuminator, and it may fairly be inferred that the special function 

 of a diaphragm as now used was not then (as it is not always now) 

 comprehended. The Brewster design has indeed been long super- 

 seded by arrangements which leave the mirror free and yet combine 

 refraction and reflection of light in pencils of any desired angular 

 magnitude applicable to central or lateral illumination as well as 

 for showing a positive image on a dark background, so that every 

 desired mode of illumination may be efi'ected without change of 

 apparatus, or of the position of light-source. But for the present 

 improved arrangements we are indebted to the practical optician 

 rather than to any general advance in theoretical knowledge. For 

 we still hear a great deal about " condensing " lenses, which never- 

 theless only increase the brightness by bringing an image of the 

 original light-source nearer to the eye by optical means, without 

 any actual increase of specific intensity. We are still taught the 

 superiority of achromatic illumination, though it is easy to control 

 by proper use of a diaphragm any excess of colour that may arise 

 from dispersion of rays given ofi' by reflection from mirrors, prisms, 

 &c. And yet we still find the diaphragm, whose special function is 

 to regulate by limiting apertures the size of the illuminating cone, 

 placed in any chance position required by the mere mechanical 

 exigencies of a thick Microscope stage ; the usual consequence of 

 which is that when an objective of even moderate power is used, its 

 angular aj^erture is far in excess of any illuminating cone which 

 can be brought to bear on the object, so that the full resolving 

 power of the objective cannot be obtained without additional appa- 

 ratus. And while it is universally accepted that light is shut off 

 by means of the diaphragm, we still find the old diagrams of mirror 

 reflection invarioMy indicating — in defiance of the fundamental 

 law of incidence and reflection — a parallel beam of light traced in 

 towards the object, whereas a moment's consideration must con- 

 vince everyone that if light is incident with imrallel rays upon a 



