ON THE THEORY OF THE MICROSCOPE 259 



** resolving power" has been credited to the objective as its 

 special function and attribute, though its niidus operandi has not 

 hitherto been satisfactorily explained. The fact that so striking 

 a physical change in the condition of the illuminating rays takes 

 place, under certain conditions, in the ohjeci itself^ and that this 

 change is entirely due to the structural constitution of the object, 

 greatly enhances the importance of suitable methods of illumination, 

 and shews us the need of a most careful study of the physical con- 

 ditions under which an object is presented to the objective. Good 

 illumination is thus not only '' half the battle," but being also the 

 Jlrst half, determines the subsequent action of the objective, for the 

 rays which fall upon the front lens of the combination (derived in 

 the first instance from the source of illumination employed) take, 

 so to speak, a fresh start from the separate constituent elements of 

 the object. A portion] of them undergoes more or less complete 

 absorption, or passing through relatively transparent parts, suffers 

 but slight change ; whilst for another portion, diffraction, under 

 given circumstances, breaks the fine pencils which fall upon 

 minute elements of the object into separate groups of rays, which 

 take a different course from the undiffracted rays, so that 

 additional images of these details are formed which appear as if 

 self luminous, whilst waves of interference are initiated which 

 occasion shadow. A.nd all this happens before the action of the 

 objective commences ; waves of ordinary and diffracted light 

 entering together, and thus potentially determining beforehand 

 the constitution of the microscope picture about to be formed. 



For the admission of rays of all degrees of angular divergence 

 from the axis of the microscope, {i.e., the whole sweep of 90"^ in 

 either side of the zenith down to the horizon) the front lens of an 

 objective must be a hemisphere, with its plane surface turned 

 towards the object, and such a lens is as simple as it is efficient, 

 *' ad hoc^ Eut in all further dealing with the light thus let in, 

 mathematical science and technical skill are needed for the 

 correction of errors of focussing function, and of chromatic 

 dispersion inseparable from the material condition of the problem 



