1891.] MICROSCOPICAL JOURNAL. 29 



In objectives provided with cover-correction the graduation is so ar- 

 ranged as to read to i-ioo mm. No matter what the power of objective 

 or whether dry or water immersion, the number gives proper correction 

 for a thickness corresponding lo it. Tlius, with a cover-glass of o.30 

 mm. the collar of such an objective need merely to be set at 3o to give 

 the proper correction, and, consequently, the best results. On the other 

 hand, with an objective which is graduated on this system the correct 

 thickness of cover-glasses can be determined by obtaining the proper 

 correction on prepai'ations previously made, but on which the thickness 

 of cover-glass is not noted and the thickness may be marked on them for 

 future convenience. To do this successfully, however, the necessary 

 experience is requisite. All the other scales give the correct tube length 

 in inches and millimeter for covers corresponding to them, and in this 

 manner offer a ready and definite means of correction. The tube 

 lengths required for the thinnest and thickest covers are so extreme that 

 probably no convenient means for obtaining them can be practically 

 arranged, but they can be so approximately if not entirely. At any rate, 

 the micrometer will detect the requirements before using the covers, and 

 those deviating considerably from the normal can be used on objects for 

 use with low powers only, in which case the effect will not be very 

 appreciable. 



In this system I do not overlook the fact that variation in tube length 

 involves a variation in magnifying power ; but, except in cases when 

 micrometers ai"e used, I consider this of secondary importance, as it 

 always is in comparison to results obtained in resolving and defining 

 power. 



This system involves four conditions : 



First. That all cover-glass be measured before using them, and that 

 the thickness be noted on the preparation. 



Second. That for convenience all draw-tubes be marked in inches or 

 millimeters, or both. 



Third. That adjustable objectives be corrected according to this scale. 



Boiirth. That the same tube length and cover-glass thickness be used 

 in all original corrections of objectives. 



As regards the first condition, there are many microscopists now who 

 measure all their covers before using them ; but- the mere knowledge of 

 thickness has been of no value up to the present time, because this in 

 itself has been no guide in obtaining better results except by approxi- 

 mation. My aim in connection with this system has been to devise an 

 instrument which possesses a high degree of accuracy and still be so 

 inexpensive that its price should be no obstacle to its general use. 



The celebrated preparer of objects, Mr. J. D. Moeller, and others 

 have kindly agreed to mark the thickness of covers in their objects so 

 as to aid the introduction of this system, and other preparers can no 

 doubt be induced to do so if its advantages can be proven. 



As regards the second point, many manufacturers now graduate their 

 tubes, and modern I'equirements demand that this should be more gen- 

 erally done. Our company intends, as soon as it can possibly arrange 

 to do so, to graduate the tubes of all its instruments. 



As to the third and fourth conditions, I cannot, of course, presume to 

 ask manufacturers to adapt their standards to this system. While it 

 will be a convenience to a large number of microscopists, I must leave 

 it to the merits this system may possess to exei't their influence in this 

 direction. 



