159 



matter of no difficulty whatever) ; and provided there be sufficient 

 space to allow the interposition of the glass on which an object is 

 mounted. 



In making observations with the assistance of the parabolic or of 

 the sphsero- annular condenser, and an object-glass of large aperture, 

 the correct adjustment for thickness of the glass covering the object, 

 a point at all times of importance to anything like fine definition, is 

 doubly necessary with this mode of illumination. 



Either of the above-named instruments may be adapted to the il- 

 lumination of opaque objects under high powers, by using a plane 

 silver reflecting surface, slipped over the front of the objective, in the 

 same way as a Lieberkuhn is commonly applied, and raising the con- 

 denser under the object, so as to get the focus correctly adjusted. 

 This, however, I consider of less importance, on account of the 

 ordinary mode of viewing objects illuminated by these condensers 

 possessing most, if not all the advantages of an examination of bo- 

 dies as opaque. * 



Those who have only occasionally glanced casually at an object 

 thus illuminated, cannot form any just notion of the full merit of 

 Mr. Wenham's suggestion, which, however, becomes more and more 

 apparent by familiarity with it. 



The various attempts which have been made to produce the same 

 effect by different means, show more strongly than words can do, the 

 fact that there is something worth an endeavour to attain, and I can, 

 from practical experience, assert most positively that a very valuable 

 adjunct to our means of investigation has been placed at our disposal; 

 and this I can do with the less hesitation, as, although a promoter, I 

 am not the originator of it, the whole merit being due in a quarter 

 which I have already indicated. 



In conclusion, I have only to remark, in order to prevent being 

 misunderstood, that I do not consider it is by superseding, so much 

 as by adding to, our present means of acquiring information, that we 

 may hope to make steps in advance. 



* Since writing the above paragraph, I have been informed by Mr. Brook that this 

 is precisely the same mode of viewing an opaque object, in principle at least, which he 

 had previously made use of. 



