M. A. AIXSLIE ON AX ADDITION TO THE OBJECTIVE 563 



These objectives have, in fact, the extremely useful property 

 of working through almost any cover-glass ; even through a 

 thin slip, if the tube-length be closed down sufficiently, which 

 in the case of these objectives is almost always possible. 



As we increase the power of our objectives, the alteration 

 required in the tube-length increases rapidly ; to give only a 

 few instances, the figures are roughly as follow for certain 

 typical objectives : 



Watson 6 mm. Holos, N.A. 0'84 . . .3*4 



Leitz No. 5 (l/5th-in.) 7*5 



Watson l/6th-in., N.A. 0'74 9D 



Zeiss " G " water-immersion .... 9'5 



Zeiss 4-mm. Apo. (without using correction collar) 13'0 



Leitz No. 6 (l/6th-in.) 14-0 



Leitz No. 7 (l/8th-in.) 20'0 



So that a typical l/8th-in. is ten times as sensitive in this respect 

 as the Zeiss 12-mm., and seventeen times as sensitive as the 

 Watson 1/2-in. " Holos." 



In passing, I must say that this seems to me to be a strong- 

 point in favour of the employment of objectives of moderate 

 power, as long as they are of sufficient excellence to stand the 

 high eye-piecing necessary to give the desired magnification. 

 The difficulty of using such a lens as the Leitz No. 7 with a range 

 of only 50 mm. in the draw-tube will now be fairly obvious. 



It might be thought that a good deal of this difference between 

 objectives of different powers is due to the N.A. of the higher 

 powers being, as a rule, greater than that of the lower ; but the 

 above figures are practically unaltered when the N.A. is reduced to 

 about 0'6 in each case, though of course the effect of incorrect 

 tube-length on the definition is not so marked as with the full 

 aperture. 



Although I am confining my attention to objectives with 

 N.A. not less than about 0'65, it must not be thought that those 

 of lower N.A. are altogether insensitive to correct tube-length ; 

 a 25-mm. objective of N.A. 0"3, for example, will not work 

 really well at any but its computed tube-length, although such 

 objectives are not very sensitive to alterations in the thickness 

 of the cover-glass. 



