The Limits of Resolution in the Microscope By F. Crisp. 969 



Almost the only rays by which the object is now delineated are those 

 near the line F. The wave-length is here shortened from • 52689 

 fi to 0-48606 yu,, and the formula therefore gives a smaller value to 

 h, i. e. the lines resolved are closer together, so that with mono- 

 chromatic light a greater number of lines to the inch can be resolved. 



The line F is at about the limit of this resolution, for although 

 theoretically the resolving power would be increased if we utihzed 

 the darker blue rays to the exclusion of the brighter, this is practi- 

 cally impossible at present, as no means are known by which the 

 bright blue rays can be stopped off, while the darker ones are 

 admitted. 



On a precisely analogous principle photography allows of a 

 still more considerable increase in resolving power. 



In the ordinary photographic process the chemical action is 

 confined to the interval between the lines Gr and H, and has its 

 maximum near the line h. This is the same thing as if we had 

 stopped out from white light all the rest of the spectrum, and were 

 working with monochromatic light of no greater wave-length. 



Taking for a point near the line h, \ = 0-40000 fj,* the 

 resolving power in the case of photography as compared with white 

 light is shown to be increased in the inverse ratio of - 52689 to 

 0-40000. 



As before stated, the table itself gives the particular hue for 

 which the column of resolving power was calculated, and the 

 possible increase with shorter wave-lengths was duly noted with 

 the first publication of the table,t but it will perhaps tend to prevent 

 in future any misapprehension if two further columns are added, 

 giving the figures of resolving power in the case of monochromatic 

 light and photography as well as white light. Mr. J. W. Stephen- 

 son, who calculated the original table, has kindly prepared these 

 additional columns, and the table will in future be printed as 

 appended to this paper. 



The case of sunlight still remains to be considered. It is un- 

 doubted that with sunlight greater resolving power can be obtained 

 than with lamplight, but the explanation is entirely different from 

 that which applies to monochromatic light and photography. 



It seems to have been supposed that by using sunlight instead 

 of lamplight we should virtually get the benefit of reduced wave- 

 length, as although the difference of relative intensity of the various 

 colours is very slight in the lower portion of the spectrum, it is 

 large in the case of the upper portion, so that the intensity of the 

 dark blue is greater with sunlight than with lamplight. This 

 Bupposition, however, overlooks the fact that in the case of white 

 light (whether lamplight or sunlight), the dark blue and violet 

 have practically no action in tlie presence of the bright blue, green, 

 and yellow. While, therefore, these colours form part of those in the 



• For ir„ X = O-.-iOGSO ^ f Sec this Journal, ii. (1870) p. 839. 



aii.l for A, A = 0- 11012 /«. 



