68 Modern Microscopy 



draw-tubes be actuated by rack and pinion, and the con- 

 venience of this arrangement cannot be too strongly urged 

 upon microscopists. Messrs. Baker, Beck, and Watson 

 and Sons have adopted it in their large models, and it has 

 evidently met with considerable appreciation, for it is now 

 made in a cheaper form by the latter firm and Messrs. 

 Swift and Sons for their students' instruments, and anyone 

 with a limited purse, but who wished to do his work in the 

 most precise manner, could have it in an inexpensive form 

 of instrument. 



DIRECTIONS FOR USING A CORRECTION 

 COLLAR AND CORRECTING BY TUBE- 

 LENGTH. 



This may be accomplished in a systematic manner if it 

 be borne in mind that the aim is to eliminate spherical 

 aberration, which defect may be defined as a difference of 

 focus between the central and marginal zones of an objec- 

 tive. Hence the correct tube-length or the best position 

 of the correction collar has been found when some strongly- 

 marked detail or outline of the object remains in exact 

 focus under any change of illumination, say from a small 

 to a large diaphragm opening beneath the condenser, or, 

 better still, by changing the illumination from central to 

 very oblique, these changes being made with great care, so 

 as not to disturb the other adjustments. 



The following process will be the safest and quickest : 

 Start with the shortest tube-length, or when there is a 

 correction collar, with the position corresponding to the 

 thickest cover-glass; carefully focus some sharp outline 

 with, say, a J central cone, then change to a f cone, or, 

 better still, to very oblique light. Unless the object — owing 

 to an exceptionally thick cover-glass, or a very badly 

 adjusted lens — is beyond the range of your adjustments, 

 you will find evidence of under-correction — that is, the lens 

 will have to be brought closer to the object with the wide 

 cone, or oblique light, than with central light. 



