178 Transactions of the Society. 



one of a lower numerical aperture and of the same amplification. 

 Professor Abbe writes, in reference to stops and diaphragms in 

 relation to high numerical apertured lenses, " The greater penetra- 

 tion and insensibihty of the low apertures may of course be attained 

 thereby, but nevertheless this device is only a makeshift, and the 

 result is inferior to that obtained by objectives originally arranged 

 for a lower aperture. The low-angled lens which is made out of 

 a good wide-angled one by means of a stop, is, in optical respects, 

 a relatively bad objective — not nearly as well corrected as the same 

 power would be if carefully adjusted for the lower angle." There 

 can be no doubt that microscopists will require most carefully made 

 objectives with a low numerical aperture as much as ever, and no 

 wise man will confine himself to nothing else in the way of ob- 

 jectives than those of high numerical apertures, whatever may be 

 the amplification. Clinching the argument, Professor Abbe con- 

 cludes his essay with the remark, " Scientific work with the 

 Microscope will always require not only high-power objectives of 

 the widest attainable apertures, but also carefully finished lower 

 powers of small and very moderate apertures." 



One of the difiiculties met with in the adjustment of objectives 

 for correcting the aberration involved by the presence of a thin 

 glass cover and a medium over the object, is caused by the awkward 

 position and direction of movement of the screw-collar. A careful 

 microscopist will of course keep the screw sufiiciently easily move- 

 able for his purpose, but unless this is done, the attempt to unscrew 

 or screw up is often accompanied by drawing the objective out of 

 the desired line of sight, and even by unscrewing it at the junction 

 with the tube. In fact it is perfectly obvious to those who have 

 the opportunity of working vdth other observers, that accidents 

 readily happen to the object during the process of correction. 

 These troubles have been noticed in every manual on microscopy. 

 It appears, however, that the worm-wheel and tangent screw, 

 suggested by Mr. J. Deby, when its application is made a little 

 easier, will meet all difficulties, and it may be added to a screw- 

 collar which could be used without the improvement in those rare 

 instances when a rapid movement is required. 



Very few microscopists care to correct their objectives during 

 ordinary work, and principally because they have not seen the 

 difi'erence made in the appearance of an object by the process when 

 it has been carefully carried out. But when an object, hitherto 

 unsatisfactorily defined, presents itself under a clear and definite 

 aspect, conversion to the opinion that there is an absolute necessity 

 for correction in all delicate investigations regarding minute 

 structures speedily ensues. There is no doubt that with very few 

 exceptions the microscopic work relating to the morphology of the 

 animal and vegetable kingdom has been conducted either without 



