A Method of Adjusting Tube Length. 121 



objective if the most is made of this method. For example, to 

 resolve Pleurosigma the zones, roughly, 0'3 N.A. on either side of 

 the central zone are used. If the central zone and these zones be 

 adjusted to be in spherical correction, then to a certain extent the 

 corrections of the other zones do not matter. 



This is the adjustment at which coarse and fine details are 

 found to focus approximately together in tlie same plane. Most 

 objectives show some spherical error in tlie outer zones. This may 

 be ignored when using the method for correcting tube length, if a 

 |-zone of illumination is to be employed, and if the object is not 

 of such a nature that the outer zones are illuminated by diffracted 

 or refracted light. Consideration shows that the principal advantage 

 of this technique over other methods is that by it the different 

 zones of the objective are separately investigated, and therefore 

 jnore information is obtained than by previous methods in which 

 the objective is treated as a whole. 



The adjustment of tube length rapidly becomes automatic in 

 practice : " motion of image with the slit " equals " plus " equals 

 "a longer tube"; "motion against the slit" equals "minus" 

 equals " a shorter tube." This method is found to be of wide 

 application, and is at present being more fully* worked out for 

 making complete measurements of the optical performance ot 

 objectives. 



The Application of the Method to the Substage 



condensek. 



This method is also found to be very useful for adjusting the 

 distance between the illuminant and the substage condenser in 

 order to eliminate spherical aberration. In my opinion it has 

 advantages for this purpose over previous methods. The tech- 

 nique may be briefly described as follows : — The same slit-shaped 

 aperture is mounted beneath the condenser as previously described 

 for the method of adjusting tube length. The objective to be used 

 is now focused on the slide and the condenser adjusted so that the 

 image of the illuminant is centred and roughly in focus. The card 

 is placed beneath the condenser mounting so that the slit is 

 roughly central with the optical axis. (This is readily done by 

 slipping out the eye-piece and looking down the tube.) The con- 

 denser is now carefully focused so that the image of the illuminant 

 is in the same plane as the slide. The card is now shifted so that 

 the slit travels at right angles to its long axis, and the image of 

 the illuminant carefully watched. In general the image will be 

 seen to move either in the same or in the opposite direction to the 

 slit. The movement of the image of the illuminant is of course 

 judged in reference to some prominent feature on the slide, in 



