COERESPONDENCE. 239 



very long period, in which, to say the least of it, after very strong and 

 arrogant language Mr. Weuham has come off second best, I have no 

 doubt the generality of your readers will think it wiser of Dr. Pigott 

 to pursue his own researches, rather than to answer insinuations 

 against his character, which might possibly be better settled else- 

 where ; at the same time, the peculiar way in which the subject has 

 been treated will be best shown by placing some of Mr. W.'s state- 

 ments within inverted commas for the reader's delectation : — 



" Dr. Pigott has devised no new method of any utility for deciding 

 such errors, and that his inferences were drawn from an erroneous 

 interpretation of the structure of known test-objects." 



To this it may be replied : — 



Dr. Pigott not only designed, but repeatedly described, tests not 

 previously employed — the double star test, &c., &c. ; and has supplied 

 means of measuring with close approximation the actual amount of 

 error produced in the apparent size of small beads by spherical aber- 

 ration of the best glasses. 



Mr. Wenham was invited to see the various apparatus employed, 

 and to witness experiments therewith, but declined. 



The inferences were not drawn, as stated, "from erroneous inter- 

 pretations of known test-objects." If Mr. Wenham had chosen to see 

 the experiments, he woxtld have been aware that Dr. Pigott did not 

 consider any of the known test-objects cajiable of affording sufficiently 

 accurate information, and he therefore set to work to devise new ones 

 about which there could be no doubt. 



" The colour test is no new featxire." 



Dr. Pigott did not claim any novelty in the colour test as indicative 

 of good performance. Dr. Goring nearly forty years ago preferred 

 glasses rather under-corrected. But it remained for Dr. Pigott to dis- 

 cover that spherical aberration at present, in adjustable glasses, cannot 

 be destroyed without disturbing the formerly so much valued achro- 

 matism. And in the August number of this Jom-nal, he for the first 

 time has shown the colour test may be employed for indicating minute 

 changes in thickness or depth of focus so as to determine planes of 

 position. 



" The mere assertion that there is a certain residuary aberration is 

 unsatisfactory, and seems to have been raised from the region of 

 phantoms, and its shadow-form is the result of a wrong interpretation 

 of structure from illusory headings." 



Mr. Wenham again chooses here to ignore the fact that Dr. Pigott 

 has indicated modes of measuring the amount of aberration, and that 

 his tests were not confined to what Mr. Wenham calls "illusory 

 headings." 



" These (headings) Dr. Pigott has great skill in displaying as a 

 reality, enhanced by drawings made by persons who may be clever in 

 ordinary use of the pencil, but clumsy and inaccurate in the delinea- 

 tion of microscopic subjects." 



Anyone desirous of truth would surely not complain that when he 



