120 Transactions of the Society. 



[If subsequently it is found necessary to employ a different 

 eye-piece, then it must either be parfocal with the one for which 

 the tube length has been corrected or suitable readjustment of 

 tube length effected. This may be easily carried out if, after 

 changing the eye-pieces, the focus is restored, not by touching the 

 iine-adjustment but by moving the draw tube. Por this purpose 

 a rack-work draw tube is an advantage, but is not essential. Small 

 tight-fitting rings of brass of the correct width may be used for 

 rendering ordinary or compensated eye-pieces parfocal. This of 

 course cannot be done in the case of holoscopic eye-pi6ces because 

 the focal plane shifts with the adjustment of compensation.] The 

 microscope is now focused with the pointer corresponding to 

 some suitable part of the slide. In the case of histological or 

 bacteriological specimens no difficulty is encountered, because 

 almost any outstanding feature may be employed. In the case of 

 diatoms a different procedure will be necessary, because the 

 structure to be examined may not be resolved when the slit is 

 placed beneath the condenser because of the small illuminating 

 •cone. If coarse structure, such as a mid-rib or a broken edge or 

 even dust particles, lie in the same plane as the structure to be 

 resolved, they may be used for the purpose of tube-length adjust- 

 ment. 



The image of some suitable portion of the object having been 

 caused to correspond with the end of the eye-piece pointer, the 

 image is carefully focused with the slit in the centre of the 

 aperture of the condenser. The slit is now moved from side to 

 side across the back of the condenser, and the image carefully 

 watched. In general the image will be found to move either in the 

 same or in the opposite direction of the slit. If in the same 

 direction, the tube length is too short, and vice versa. 



If the microscope possesses a mechanical draw tube, the correct 

 adjustment is very quickly made, for it is then found that with a 

 good objective the image is practically stationary. In fact, in my 

 opinion, the l)etter the objective the more definitely and rapidly 

 can the adjustment be made. Now, in a poor objective it may be 

 found that under no conditions will the image remain stationary ; 

 for example, as the slit is moved from the centre outwards, the 

 image may be found to move first against the motion of the slit, 

 then back to its old position, and then over to the other side. This 

 shows that (for this setting of the draw tube) there is over-correction 

 of the ^-zone, correct correction of the ^-zone, and under-correction 

 of the f-zone. A longer setting of the tube will give increased 

 movement and therefore increased over-correction of the ^-zone, 

 -over- correction in the ^-zone, and correction in the f-zone. A 

 shorter tube corrects the ^-zone at the expense of ^ and |. It is 

 thus impossible by adjusting tube length to eliminate the spherical 

 aberration. Much good work can be done even with such an 



