190 On Zeiss' ^^ih Immersion. By W. J. Hickie 



" Let us then inquire what will be the effect of a slight 

 displacement of the combinations, whereby these, though remaining 

 in the same plane, are pushed aside laterally in any direction." 



The reader will see on pp. 70-74 of the work referred to how 

 the editors have carried out their demonstration. 



They further remark : " With every increase of displacement a 

 corresponding marginal jjortion of the object is effaced, until at 

 length the entire margin suffers more or less, leaving only a 

 central portion of the field still retaining its original clearness, 

 which portion, however, may take any eccentric position." 



Their final summing up is : " That imperfect centering does, 

 indeed, exert an injurious effect upon the optical image, but that 

 this injurious effect limits itself (if we except the case where the 

 axes of the combinations are not coincident with the axis of the 

 tube), to the margin of the field, and only reaches the centre when 

 the defects of centering are abnormally excessive." 



Therefore the effects of imperfect centering are " to limit the 

 area of definition," not to materially affect its absolute quality. 



But this perfect centering is the product of several factors, and 

 depends upon the fulfilment of more conditions than one. Granted 

 that the axes of all the combinations are exactly coincident, there 

 will still remain the exact coincidence of these with the axis of the 

 tube, — which may be affected by the form of the tube itself, or even 

 by the screw of the adapters, — and next, the coincidence of all the 

 above with the axis of the eye-piece employed, — which latter, again, 

 may be affected by looseness of fit or indifferent workmanship. 



It may, indeed, happen that displacement in one direction 

 is balanced by displacement in another direction; but these are 

 elements that cannot generally be taken into account. Where the 

 second defect comes into operation the injurious effects increase 

 proportionately with the lengthening of the tube. As this defect 

 sets the optical axes of the combinations at a slight angle with the 

 axis of the tube, its immediate result is to introduce " want of 

 parallelism," which is infinitely more destructive of good definition 

 than imperfect centering. Anyone may demonstrate this to him- 

 self by looking at an object with a hand-magnifier, and holding the 

 instrument slightly tilted. It will be noticed that I have omitted 

 a case of possible occurrence ; that is, where the lenses themselves 

 are tilted in the setting. This, however, is not what is commonly 

 included in the popular idea of " bad centering," but approximates 

 to " want of parallelism. What is tested, then, by deep eye-piecing 

 is " want of parallelism '" plus chromatic and spherical aberration. 

 Nageli and Schwendener (p. 139) remark, in reference to this, that 

 " coarse, blurred outhnes, and coloured rims, which do not belong 

 to the object, invariably point to defective correction of chromatic 

 and spherical aberration; ' and that "defective centering is betrayed 



