66 



ing on the spherical correction of objectives, one is 

 astounded by the difference in standard cover- glasses 

 used by different makers in correcting non-adjustable 

 objectives. With a thickness of 0.10 mm. for the 

 thinnest and 0.25 mm. for the thickest, it is only too 

 apparent that with the additional variation in lengths 

 of tubes it is beyond the power of the microscopist to 

 obtain even approximately the best results from his 

 objectives. More than this, a large quota of the 

 advance made in recent years in the capacity of objec- 

 tives has been lost. 



"The greatest difficulty is met with non-adjustaleb 

 objectives. As is well known, compensation for thick- 

 ness may be obtained in the proper adjustment of 

 tube length ; but while not all microscopes are suit- 

 ably provided with draw-tubes, the requisite exper- 

 ience and skill is lacking with a large number of 

 microscopists to properly make the correction in this 

 manner, as well as in objectives specially provided 

 with collar correction. I am sure that microscopists 

 of long experience will bear me out in the statement 

 that results with adjustable objectives depend upon 

 individual skill, and that many such objectives now 

 in use fail to give results corresponding to their 

 capacity. It would seem, therefore, that any system 

 to permit the full utilization of the capacity of objec- 

 tives should depend on no personal factor in fact, 

 should be mechanical. 



" In an objective corrected for normal thickness of 

 cover-glass there will be spherical over-correction with 



