1870.] Dr. Royston-Pigott on an Aplanatic Searcher. 329 



idea of searching the axis of the instrument for aplanatic foci, and that 

 many such foci would probahly be found to exist iu proportion to the 

 number of terms iu the equations (involving curvatures and positions). 



E. The law was then ascertained that power could be raised, and defi- 

 nition intensified, by positively correcting the searching lenses in proportion 

 as they approached the objective, at the same time applying a similar 

 correction to the observing objective. 



The chief results hitherto obtained may be thus summarized. 



The writer measured the distance gained by the aplanatic searcher, 

 whilst observing with a half-inch objective with a power of seven hundred 

 diameters, and found it two-tenths of an inch increase; so that optical 

 penetration was attainable with this high power through plate-glass nearly 

 one quarter of an inch thick, whilst visual focal depth was proportionably 

 increased. 



The aplanatic searcher increases the power of the microscope from two 

 and a half to five times the usual power obtained with a third or C eye- 

 piece of one inch focal length. The eighth thus acquires the power of a 

 twenty-fifth, the penetration of a one-fourth. And at the same time the 

 lowest possible eyepiece (3 -inch focus) is substituted for the deep eye- 

 piece formed of minute lenses, and guarded with a minutely perforated 

 cap. The writer lately exhibited to Messrs. Powell and Lealand a bril- 

 liant definition, under a power of four thousand diameters, with their new 

 " eighth immersion " lens, by means of the searcher and low eyepiece. 



The traverse of the aplanatic searcher introduces remarkable chromatic 

 corrections displayed in the unexpected colouring developed in microscopic 

 test-objects*. 



The singular properties or, rather, phenomena shown by eidola, enable 

 the practised observer in many cases to distinguish between true and 

 delusive appearances, especially when aided by the aberrameter applied to 

 the objective to display excentrical aberration by cutting off excentrical 

 rays. 



Eidola are symmetrically placed on each side of the best focal point, as 

 ascertained by the aberrameter when the compensations have attained a 

 delicate balance of opposite corrections. 



If the beading, for instance, of a test-object exists in two contiguous 

 parallel planes, the eidola of one set is commingled with the true image of 

 the other. But the upper or lower set may be separately displayed, either 

 by depressing the false eidola of the lower stratum, or elevating the 

 eidola of the upper; for when the eidola of two contiguous strata are in- 

 termingled, correct definition is impossible so long as the aperture of the 

 objective remains considerable. 



One other result accrues : when an objective, otherwise excellent, cannot 



* Alluded to by Mr. Reade, F.R.S., in the ' Popular Science Review ' for April 1870. 



2 c 2 



