A. E. CONRADY 65 



from the mounters' lathes. The tools and methods employed in 

 really manufacturing lenses on this system were shown by Messrs. 

 W. Watson and Sons, Ltd., at the exhibition at King's College in 

 January, 1917, and will be found described and illustrated in the 

 record of that exhibition. 



Ill old English practice the component lenses of microscope 

 objectives and condensers used to be fixed in their cells by cement 

 of the sealing-wax type. Many old lenses which are still found in 

 perfect adjustment 50 or more years after being mounted demonstrate 

 that the cement may hold the lenses in correct position almost inde- 

 finitely : but other experiences, especially with lenses used in tropical 

 countries, suggest that shifting may occur, and it is therefore to be 

 strongly urged that all microscope lenses should be held between 

 metallic shoulders at both ends by being bezelled into their cells, 

 care being naturally required to avoid pressure and distortion through 

 too tight a fit. 



A few words may usefully be addressed to the users of microscope 

 objectives. All the higher powers are very sensitive (the more so 

 the more perfect the spherical correction) to the thickness of the 

 coverglass ylus any mounting niediuni intervening between object 

 and coverglass, and also to variations of tube-length, and the best 

 result can only be obtained by adapting the tube-length (or the 

 adjustment of the correction-collar if there is one) to the individual 

 coverglass. It is grossly unfair to interchange one objective with 

 another of similar power but different make on the tube-length 

 suiting the objective treated as the standard and then to condemn 

 the new objective (usually an English one !) because it gives an 

 obviously inferior image. It is not even fair merely to find the best 

 tube-length for the new objective, for if the change of tube-length 

 is considerable and in the direction of lengthening, the total mag- 

 nification will be much higher and the image correspondingly duller 

 and more fuzzy. To make the comparison fair, each objective should 

 be tried at its own best tube-length, and with such an eye-piece as 

 to give practically the same total magnification. 



Another point on which users of objectives err to their own 

 detriment is an excess of faith in numerical aperture, I have heard 

 microscopists boast of possessing an objective, say, of 1.43 N.A., 

 whereas somebody else had one of only barely 1.40, and a careful 

 test would show that whilst the 1.43 was an indifferent lens, the 

 1.40 was excellent. The fancied advantage of 2 per cent., then, is 

 really a disadvantage of perhaps 25 per cent, or more. 



One of the few disservices which Abbe did to microscopy was 

 the pushing of the N.A. of dry lenses to .95 and to a lesser extent 

 the increase of that of oil-lenses to 1.40. The extreme marginal zone 

 of the apochromatic dry objectives of .95 N.A. is particularly badly 

 corrected, so much so that the lenses will only bear a solid illumin- 

 ating cone of about .65 N.A. even on the Abbe test-plate, and that 

 with annular light bringing only the marginal zone into action 

 correction-collar and tube-length combined do not allow of reaching 

 a point of good spher^'cal correction. There is no doubt that Abbe's 

 own earlier dictum still holds, to the effect that beyond about .85 

 N.A. the higher aberrations become unmanageable unless the free 

 working distance is reduced to a very few thousandths of an inch. 



