156 



My thanks are justly due to Messrs. Powell and Lealand, and 

 to Messrs. Smith and Beck, for the accuracy with which they have 

 constructed the apparatus according to the drawings I furnished 

 them with. 



When this species of illumination is adopted, whether by means 

 of the parabolic or my own condenser, the aperture must of necessity 

 be greater than that of the object-glass, in order to produce the black 

 field, the rays of light being thus prevented from entering the objec- 

 tive, unless previously affected by the interposition of an object ; and 

 I find in practice a direct corroboration of a point which had previ- 

 ously occurred to me in theory, viz., that the best effect is produced 

 when the aperture of the condenser is but a few degrees greater than 

 that of the object-glass with which it is intended to be used. This 

 is a fact of which I have no doubt whatever. The explanation I 

 take to be as follows : — When viewing an opaque object, the light 

 thrown upon it is partly absorbed and partly reflected, or, as some 

 would call it, radiated, that is, reflected in all directions, and not as 

 a polished surface reflects : provided it be understood what is meant 

 by this term radiated ; winch is a convenient one to designate the 

 kind of reflection indicated. It is therefore by this radiated light 

 that the eye is stimulated in viewing an opaque object ; and the more 

 vertically the light is thrown, the better will be the illumination, ow- 

 ing to the fact, that the same quantity of rays have a larger surface to 

 illuminate when thrown obliquely than when falling vertically, the 

 light being more and more diluted as the obliquity increases. 



Now with the black field illumination, although the light is trans- 

 mitted through the object, in this case also a portion of it is absorb- 

 ed, and (unless the surfaces of a homogeneous structure be parallel 

 and polished) the remainder is refracted radially, so that, in point of 

 fact, it differs but little, if at all, from what we have agreed to call 

 radiant light in an opaque object; the one being reflected in all di- 

 rections, the other refracted in the same way. 



Although a certain amount of obliquity in the incident ray is ne- 

 cessary to produce the dark field, yet in order to obviate any need- 

 less dilution of the light, it is highly desirable that the obliquity 

 should not exceed what is absolutely requisite, particularly as those 

 objects which exert the least amount of force in altering the direc- 

 tion of the rays, are precisely those in which the extra assistance to 

 definition afforded by this species of illumination is most required, 

 and consequently the directive force should be tasked as little as 

 possible. 



