^JoSnuf)/june°ri87a'] PROCEEDINGS OF SOCIETIES. 325 



pliotograjihs, made uiifler my sixpervision, wliicli I contributed to the 

 Microscoi)ical Society some time since. 



Will you not, my dear sir, read this letter and exhibit the accom- 

 panying photographs at the next meeting of the Microscopical Society ? 



I have the honour to be, 



Very respectfully, 



J. J. WOODWABD, 



Assist. -Surgeon and Breret Lt.-Col. U.S.A. 



Mr. Slack remarked that the effects produced by Dr. Pigott had 

 no connection whatever with the mode of illumination adopted by 

 Dr. Woodward. Dr. Woodward's method of removing the condenser 

 and using the direct light of the sun reflected through the object was 

 objectionable, as it was well kno^\'n that if a blaze of light is sent 

 into the microscope false appearances are exhibited. Dr. Pigott's 

 mode of illuminating is entirely different. He can show the per- 

 formance of the finest objective of Powell and Lealand's in the ordi- 

 nary way ; and by introducing an apparatus of his own, for correcting 

 residuary aberration, the definition of kno^vTi objects is improved, and 

 the headings of the Podura scale are inmiediately seen. Ho had spent 

 a long evening with Dr. Pigott, who had performed a number of experi- 

 ments with lenses which left no doubt on his (Mr. Slack's) mind that 

 the best of them produce spherical aberrations. 



A very striking experiment was made by converting the mici'o- 

 scope into a telescope, and viewing with the objective to be tested 

 artificial double stars ingeniously contrived by Dr. Pigott, and con- 

 sisting of minute discs of light, the diameters and interspaces of which 

 were both known. A badly-corrected glass would not divide them at 

 all, and a well-corrected glass of Powell and Lealand's, which just 

 divided them, gave smaller discs and larger interspaces — thus coming 

 much nearer true definition — when Dr. Pigott's correcting apparatus 

 was introduced. 



Mr. J. Beck inquired whether Dr. Pigott had used a simple glo- 

 bule of quicksilver in order to correct the faults that arise from 

 spherical aberration, and whether in his method of illumination either 

 outward or inward coma was apparent ; and also whether the rings of 

 light expand evenly, or whether they expand on one side of the focus 

 only. 



Mr. Slack said that Dr. Pigott had used globules of mercury, but 

 did not consider them so severe a test as others he had devised ; as it 

 was well known to mathematicians that these globules were not per- 

 fectly spherical, and they were not illuminated by dii'ect light. 



The President v/ished to confirm Mr. Slack's observations upon 

 Dr. Pigott's discoveries. In answer to Mr. Beck, he stated that 

 Dr. Pigott does show bright points of light in concentric rings on 

 both sides of the focus. Other experiments clearly prove that he does 

 very materially diminish spherical aberration. The flame of a lamp, 

 for instance, under a really good object-glass, is seen to be surrounded 

 with a halo of light ; but as soon as the " aplanatic searcher " is intro- 



