ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 857 



elements, the advantage to be expected by the use of the correction- 

 collar is not only absolutely illusory, but that it is attended with 

 many serious disadvantages. 



In the observation of diatoms, which one has seen so often, and 

 the structure of which is so simple and characteristic, it is not a 

 matter of great difficulty to find approximately the best correction by 

 experiment, since one forms a judgment from the clearness and dis- 

 tinctness of the image. For those, therefore, who study preferably the 

 structure of diatoms, or who have set themselves the task of demon- 

 strating test-objects a number of times (from whom has originated the 

 desire for this contrivance), the correction-collar may prove of some 

 slight advantage in the sharpness of the image, and for that reason may 

 appear to be a desirable requisite. For this class of observation it may 

 be readily admitted that at least no important disadvantage can arise. 



For histologists, however, the case is very different. With the 

 objects that come under their observation, especially if they are of 

 very delicate and complicated structure, it is almost impossible to find 

 the best correction by mere trial. In endeavouring to find the " best 

 image " we are just as likely to arrive at a completely false correction 

 (which produces false images) as upon the proper one. The widest 

 latitude is thereby given to every possible subjective fancy and false 

 arbitrary interpretation, and those deviations from the best correction 

 which still remain, in the in other respects skilful use of objectives 

 with fixed mounting and carefully corrected for a given length of the 

 tube and a particular immersion fluid, are perfectly insignificant and 

 harmless as compared with the great uncertainty and grave aberra- 

 tions which the use of the correction-collar introduces. 



If the best correction for a certain thickness of cover-glass is 

 required, there is only one object by means of which this can be 

 obtained with perfect certainty and with the smallest amount of sub- 

 jective fancy, so that correct images of all objects of any structure can 

 be guaranteed (without false differences in level, &c). This object is 

 the Abbe test-plate, by which the correct co-operation of all zones of 

 the aperture can be judged of. That, in comparison with this plate, 

 the structures of the valves of the Diatomaceaa are by no means suffi- 

 cient, is best proved by the case brought forward by Dr. Edwards 

 Smith, in which personal equation evidently played no unimportant 

 part. And if even in the case of such an object — striae of diatoms 

 of well-known nature — such latitude was given for the exercise of 

 personal caprice in the adjustment of the correction-screw for the 

 " best image," how great may it be when we are dealing with unknown 

 delicate and complicated structures ? Under such circumstances, how 

 easily may the employment of the correction-collar become rather a 

 subject of misuse than of use ? With high power dry objectives and 

 water-immersion objectives the correction-collar is a necessary evil 

 which must be endured. Where, however, it can be dispensed with, 

 it would be folly to retain it on account of entirely subordinate and 

 unimportant advantages. Especially may it be very decidedly rejected 

 in all scientific work with homogeneous-immersion objectives. The 

 slight restriction in the use of objectives with fixed mounting can the 



