PROCEEDINGS OF SOCIETIES, 177 



between the eye-piece and object-glass. This pursuit he assured the 

 meeting was an arduous one, there was the focussing, then the length of 

 tube, the position of the searcher, and the adjustment of the screw- 

 collar, and they knew how many changes could be got out of four 

 bells, but here were four quantities, each capable in itself of very great 

 variation, so that it might be judged how much was involved in 

 making these investigations. One of the most pleasing results of his 

 labours was found whilst examining one of Mr. Slack's silica films. 

 In working one day upon one of these slides he found that in par- 

 ticular positions of the screw-collar of the searcher and of eye-piece 

 those cracks which were so red before now became beautifully black, 

 so that the searcher corrects chromatic aberration also. The subject 

 was so intricate and difficult that he should recommend no one to 

 take up the searcher, unless they were prepared to meet difficulties 

 at every point. He hoped soon to be able to show a combination 

 which gave to a quarter foiu" times the usual power under the use 

 of a low eye-piece instead of an uncomfortable deep one. Messrs. 

 Powell and Lealand might very likely be disgusted to hear that he 

 often used their gi^-inch at five inches distance, instead of ten ; of 

 com'se, as already stated, it quite * under-corrected the lens, but he 

 got a brilliancy which astonished him. If any gentleman would take 

 the trouble of getting a tube made like the one he held in his hand, 

 and would commence with the first piece and go on building it up 

 as he had described, they would see such a beautiful series of effects 

 as would well repay them the cost, 10s., expended upon it. He had 

 never found that an objective ever worked really well with any 

 other covering glass than that for which it was originally made. 

 Could anybody tell him what " covering glass " was made of ? He 

 thought it so important that he had constructed an elaborate instru- 

 ment (a drawing of which he exhibited) to measm-e its refraction. 



Mr. Wenham said he believed it was a light flint. 



Dr. Pigott : Yes, it was the lightest flint glass that could be found. 

 He then drew attention to a drawing of a new refractometer on paper, 

 showing a method of obtaining the refractive index of thin plates of 

 glass, and he had found that its refractive index was 1 • 555 for mean 

 rays. 



The President thought that as a means of obtaining the refractive 

 index this plan was so important that he hoped Dr. Pigott would 

 make it the subject of a paper. 



Dr. Pigott explained the principle upon which the instrument was 

 based. He said when a piece of glass was placed over any object it 

 caused the image to rise, and this effect being proportionate to the 

 refractive index of the glass it was only required to measure the 

 distances with and without the covering glass, and the refractive index 

 could then be obtained with great exactness. 



Mr. F. H. Wenham said he should like just to say a word or two 

 upon three points touched upon by Dr. Pigott. Fii-st, with respect to 

 the Podura scale-markings, the subject had been so controversial that 

 it was perhaps hardly proper to speak of it as ono in which this 



* Compensated, of course, by separating the lenses by the scrcw-collar. 



