176 PROCEEDINGS OF SOCIETIES. 



was auotber propositiou, that tlie aberratious as well as tlie aperture 

 of the lenses depended on this action. Why was it that, if they 

 approached or receded the front lenses, they corrected the aberration ? 

 AVhy was it? If anyone disj^uted it, it could be referred to com- 

 petent judges. If they took the collar and put it to its farthest point, 

 they would find that there was a different diameter to the pencil of 

 i-ays which passed to the back lenses, and they would also find that 

 there was a different aberration. The aberrations were disturbed by 

 altering the screw-collar, and the cause was that the back lenses 

 were then engaged with a larger or smaller pencil of rays. 



With regard to the principle of the searcher, if tliey got a tube 

 made up of several short pieces, like one which he held in his hand, 

 they could begin with one piece and go on building it up as required. 

 If they shortened the tube the result would be to violently under- 

 correct the objective lenses, and if they now formed the object at 5 in. 

 from the stage instead of 10 they would find the apertm-e diminished. 

 Now if they put a lens in the tube at different points between the 

 eye-piece and object-glass in a longer tube than usual, they would 

 find that it had different effects according to its position. The actual 

 diameter of a pencil entering the seai'cher did not exceed -J^ of an 

 inch ; the consequence was that any ordinary over-corrected lens 

 would work with it. He found the back lenses of Powell and 

 Lealand's inch object-glass made with three sets of lenses formed a 

 very good searcher. 



The President thought he should like to ask Dr. Pigott a question 

 as to his " searcher." In his communication he sjioke of the effect 

 of altering the adjustment screw, and described it as altering the 

 angular aj^erture. That was unquestionably so, but though it was 

 a feature it was not the only object to be attained. He thought 

 that the mode in which the relations were altered by the progression 

 or recession of the front lenses from the combination was laid doAvn 

 by the late Andrew Ross with very great clearness some years ago, 

 and he should like to ask Dr. Pigott whether the action of the 

 aplanatic searcher could be laid down diagrammatically in the same 

 clear manner ? 



Dr. Pigott, in reply, said that if he was desired to draw a diagram 

 he should be very pleased to do so. He then drew two upon the 

 black-aboard and explained them whilst they were in progress, showing 

 changes in aberration with variation of the position of the searcher 



has a much larger angular aperture when tlie lenses are closed up than when 

 fully open, and the aberrations are proportionately changed. Probably one- 

 hundredth of tlie turn of its collar does not chunge the linear aberration more 

 than tlie fifty-thousandth part of an inch ; nor the angular aperture more than 

 a minute proportion of a degree. But he states in one of his contributions the 

 exact amount of change (overlooked in England) in the angulur aperture for 

 opening or closing the lenses fully. Using a thin cover micrometer, I have just ascer- 

 tained that Tones' glass magnifies with 2-inch eije-piece as folloivs : — 350 diameters with 

 fully open lenses, 470 where they are fully closed. Dr. Woodward (November Journal, 

 p. 214) states that a Tollea' -,Lth has a balsam angle of 65° at the open point, and 

 when the screw-collar is fully closed it becomes ST. The change of aperture by 

 change of screw-collar has not apparently received sufScient attention in this 

 country.— G. \V. li. P. 



