ON THE THEORY OT THE MICROSCOPE. 251 



power" in its more general attributes. It only gives the limit of 

 resolution, and therewith establishes a fact which may have a cer- 

 tain value in itself, on account of the singularity of the case, but 

 which has no direct connection with the general performance of 

 the lens. 



Not can the test of *' resolving power " by direct light 

 be estimated at a much higher value. In the neighbourhood of 

 the limit of resolution corresponding to this form of illumination, 

 all direct light passes through the central zone, and all diffracted 

 light through the peripheral zone of the aperture. Independent 

 of the circumstance that residual aberration can be thrust into 

 some non-operative middle region by help of a correction collar, 

 the fact of resolution; depends, even in this case, essentially 

 upon the action of the peripheral zone, because there always lie, 

 at least two, if not more, oppositely situate diffractive pencils in 

 the periphery, which, with even indifferent co-operation of the 

 direct rays, render details visible. 



From the point of view presented by the theory here pro- 

 pounded, another method offers itself, which, while employing 

 the usual tests, brings directly into light the particular points 

 which mainly influence the quality of performance during 

 ordinary use of the microscope. If it be desired to test, in 

 a most critical^ way, the conditions of exact co-operation of 

 pencils of light which pass through every part of the aperture, 

 there are truly no better means than those afforded by natural 

 objects of the Diatom class and' insect scales, provided that the 

 mere fact of accomplished '' resolution " is not made the chief 

 consideration, but that the exact constitution of the total image 

 produced by the objective is studied. 



If, for the test, an object be chosen whose fineness of detail is 

 such that the objective to be tested just enables it to be seen wdth 

 direct illumination, [and therefore without any difficulty with 

 oblique light, it can be made, without any further preparation, to 

 bring to view the sensitive track of light through the microscope, 

 the production of which is effected by the mode of testing 



