1891.] MICROSCOPICAL JOURNAL. 27 



say that while for a long time I had felt that a system that would per- 

 mit the full utilization of the optical capacity of objectives of different 

 makers under varying conditions of cover-classes was desirable, I was 

 then forcibly impressed with the absolute necessity of a plan which 

 would offer this advantage. Outside of the differences of the lengths 

 of tubes used by different makers, which is also of great bearing on the 

 spherical correction of objectives, one is astounded by the difference in 

 standard cover-glasses used by different makers in correcting non-ad- 

 justable objectives. With a thickness of o.io mm. for the thinnest and 

 0.25 mm. for the thickest, it is only too apparent that with the addi- 

 tional variation in lengths of tubes it is beyond the power of the mi- 

 croscopist 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 objectives has been lost. 



As Professor Gage states, " A uniform thickness for cover-glass for 

 non-adjustable objectives seems also desirable," and this would be the 

 easiest solution of the question ; but while on the one hand the makers 

 of objectives have not yet agreed to use one standard on account of the 

 technical difficulties involved in departing from their established pre- 

 cedent, on the other the microscopist would hardly be willing to bear 

 the expense which would be occasioned by the loss of cover-glass not 

 conforming to the standard in order to use those of one thickness. This 

 expense might be greatly reduced by using selected covers of one 

 standard on objects for all medium and high power objectives and the 

 balance on all other preparations on which only low powers would be 

 used, but this would of course be of little avail in face of the fact that 



P manufacturers follow no standard. 

 [ The greatest difficulty is met with non-adjustable objectives. As is 

 well known, compensation for thickness may be obtained in the proper 

 adjustment of tube length ; but while not all microscopes are suitably 

 provided with draw-tubes, the requisite experience 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 — and this I have followed out in the system I shall explain. 

 In an objective corrected for normal thickness of cover-glass there 

 will be spherical over-correction with thick covers and under-correction 

 with thin covers, the amovmt of correction varying in a different ratio 

 to the amount of variations from the normal thickness. The chromatic 

 correction will also lose correspondingly, but to not so high a degree. 

 While a deviation of a few hundred millimeters in either direction will, 

 perhaps, not signify, that which occurs in covers classified in price-list 

 under one number is sufficient to seriously affect and the high powers 

 totally obliterate the definition which under normal conditions it may 

 possess. The microscopist is therefore not obtaining such resvilts as his 

 objectives ought to enable him to obtain, and the efforts of the con- 

 scientious optician to provide classified objectives of reliability and 



