516 Transactions of the Society. 



therefore, too miicli to affirm that a right understanding of optical 

 effects (initiated hefore the specific function of the objective is 

 called into play) would have sufficed to expose the weakness of the 

 strangely inadequate theory of the Microscope indicated in our 

 handbooks by the hypotheses of penetrating and resolving powers ; 

 and that the maxim not long since considered unassailable, " No 

 resolution without magnifying power," requires qualification in 

 many important particulars. 



The foregoing observations confirm to a large extent a state- 

 ment made by Mr. Crisp in a communication read last year before 

 the Society,* which I take the liberty of quoting here : — " At 

 the present time," says this gentleman, " there is nothing extant 

 which constitutes a commencement of a systematic theoretical 

 treatment of the subject of illumination ; yet, being purely optical, 

 it is obviously capable of being so treated, and great practical 

 advantages would undoubtedly follow from the development of 

 an exact theory on the subject. In nothing has the ingenuity of 

 microscopists been more exercised than in the invention of novel 

 modes of illumination for lined objects ; but however clearly these 

 appliances may bring out particular appearances, there is good 

 reason to believe that in the majority of cases they are illusions, 

 originating in the character of the illumination employed, and tbat 

 all possible methods of illumination may be reduced from the fifty 

 or more kinds now existing to less than half a dozen at the most." 



In adding to Mr. Crisp's milder denunciation a charge of erro- 

 neous teaching in respect of " the scientific illumination of objects " 

 so strenuously contended for by Sir D. Brewster, I am fully 

 sensible that such a charge requires to be sustained, not merely by 

 the evidence above adduced of contradictory opinion and practice, 

 as well as absence of definite rule, which prevails at the present 

 time, but also by souie alternative exposition of facts based on demon- 

 stration as well as theory. Such an exposition may be found in 

 the respective essays of Professors Nageli and Schwendener, Abbe, 

 Helmholtz, and others, but as it seems to be little known or appre- 

 ciated in Enghsh microscopy, an attempt to give some account of 

 what is authoritatively taught abroad as a scientific theory of illu- 

 mination, may serve at least to remove the reproach of inattention 

 on the part of English microscopists to the systematie study of so 

 important a subject. And it may be well also that the student 

 should reahze more distinctly how much of the actual performance 

 of his Microscope depends upon the scientific application of his 

 illuminating apparatus, even if he work with such simple illumina- 

 tors as the plane and concave mirror. 



It need scarcely be remarked that if the Microscope could 

 always be turned towards a bright expanse of sky or reflected 



* This Journal, i. (1878), p. 126. 



