224 Optical Curiosities of Literature. 



known and familiar, though not in the very first rank. For, I 

 ought to add, none of these incidents occurred in connection with 

 any of the three historic great houses of which England is so justly 

 proud, with all of which, so far as my experience has enabled me to 

 judge, optics means something more than mechanical work. 



The anomaly which so much perplexed me when new to the 

 work, I believe that I can now perfectly account for. Optics, in a 

 special sense, involves both principle and practice, the combination 

 of the two being necessary at almost every step, so much so, that 

 this department of knowledge cannot with propriety be called either 

 an art or a science. But in certain cases the scientific part is 

 reducible to fixed measurement and figures, and in this form can be 

 handed over to the workshop. Some opticians have in this way 

 acquired not the Science but the Art, working under others from 

 whom they have received the figures ; afterwards working the same 

 figures on their own account. Their glasses are thus made to 

 formula, the makers remaining in total ignorance of the " why " 

 and the '' wherefore " in which the formulae originated. They 

 work to these, and within certain narrow limits work well. It is 

 only when something new is required, some modification of existing 

 designs, that the deficiency is made apparent, and they are, in a 

 literal sense, thrown out of their reckoning. 



The curiosities I have been speaking of are not, however, by any 

 means confined to the literature of conversation. Feats of learning 

 which may well bear comparison with them are to be found in pub- 

 lished works — works which have a name and a certain measure of 

 authority. I will take an example from the treatise of Dr. Lardner 

 in the ' Museum of Art and Science.' In vol. vi. the question of 

 Nobert's lines is taken up, in connection with the reports of the 

 juries of the Exhibition of 1851. The juries, it seems, had 

 reported that while certain moderate powers sufiiced to resolve the 

 lower bands, to resolve the upper ones powers not only higher, but 

 very much higher, were found to be necessary. Here the writer's 

 calculating power discovers " a mistake." He counts the bands so 

 as to find the proportion which the closeness of one band bears to 

 the closeness of another. The higher band is, suppose, five times 

 as close ; therefore, a magnifying power exactly five times greater 

 must, he says, resolve the higher band. For is it not plain, demon- 

 strable mathematically, that this will present the fines so as to sub- 

 tend the same angle which in the first band was found sufficient for 

 their separation ? And in fact, says the author, proceeding to 

 generalize, we never need consult our microscopes at all to know 

 what amplification will separate a given band. Ask how many 

 lines it has to the inch, take the proportion of this to any band 

 already resolved, and you have the required amplification at once ; 

 a simple sum in Eule of Three. Finally, the writer expresses him- 



