'j°uma\KT.Tm] Remarks on High-imver Definiiion. 303 



rather less tlian the thickest covering over the objects that it is to be 

 employed ujDon. 



We have here, in the immersion lens, gone back to the original 

 condition of again adding thickness to the front, and the object may 

 now be considered under view as an uncovered object. ISiot either 

 the ivater or glass-cover has introduced a single new element of 

 correction, and will not therefore bear out the following assertion in 

 the paper referred to: — " The extraordinary difference between the 

 performance of the hydro-objective and of the pneumo-objective (the 

 plate of air and water making enormous differences in the aberra- 

 tions of the glasses) must make it apparent to ordinary common 

 sense that our old-fashioned glasses are wrong somewhere." 



One advantage ui the immersion objective is, that it almost 

 prevents the loss of hght from the reflexion of the upper surface 

 of the cover and front of lens, and in part neutralizes any error of 

 figure or polish that may exist between them. There is also another 

 condition annexed, it has the singular property of a fi'ont lens of 

 adjustahle thickness, and therefore can be set to the utmost nicety to 

 balance the aberrations. Of course there is no optical advantage 

 attendant upon the use of ivater. If a medium of the same refractive 

 power as the glass were to be employed the result would be better. 

 Water having a low refractive index, an adjustment is required for 

 each thickness of cover, and a difference of adjustment is not so 

 marked and sensitive as in the ordinary dry objective ; but if a 

 medium of similar refraction to the glass were to be used, no adjust- 

 ment would be required for any thickness of cover, supposing the 

 test-objects to be mounted thereon (which tLey generally are), for, 

 in fact, we should then view them all with a front of the same 

 thickness— considering the cover, the front lens, and the interposing 

 medium as one. 



Having now given some reasons for repudiating the persistent 

 error assumed to exist in all our best object-glasses, I must of course 

 notice the observations upon which the assurance has been founded. 

 The author of the essay expresses his opinion that this " minute 

 structure of the Podura affords the most severe trial for residuary 

 aberration with which he is acquainted." I have three ^V^h object- 

 glasses, and it is most easy to produce the beading with the worst 

 of them. The highest eye-piece should be used, the di-aw-tube 

 lengthened, and the object placed slightly out of focus. The illu- 

 mination (with the achromatic condenser) requires long and careful 

 coaxing to obtain the illusion. Figs. 4 to 7 in Dr. Pigott's paper 

 do not fairly represent the appearance. The beads are neither so 

 closely packed or so regular as there shown. The under-beads may 

 appear to cross either to the right or left, according to the illumina- 

 tion ov fancy of the observer. Having got the beaded form developed 

 to the best advantage, if we now remove the highest eye-piece and 



