854 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the object are followed) at length, without the spectator's knowledge, 

 mingle with the fluid above employed for the lens, and thus 

 destroy the whole object of search and study. This fact, then, 

 makes air angles of the highest importance, and I hope the highest 

 results have not yet been attained with them. In the main, then, I 

 agree with Abbe." 



At the Montreal Meeting of the American Association for the 

 Advancement of Science, Dr. W. B. Carpenter gave an address on the 

 practical and theoretical results in the history of the Microscope, in 

 which he dwelt mainly upon the question of wide aperture and high 

 power eye-pieces. 



Correction-adjustment for Homogeneous-immersion Objectives.* 

 Dr. L. Dippel has already briefly published his objections to the use 

 of a correction-collar in the case of homogeneous-immersion objectives 

 (in opposition to the contrary opinion of Dr. H. Van Heurck) which 

 he considers to be an abandonment of the practically most important 

 advantage for scientific work which homogeneous-immersion has 

 brought us, but is now led to return to the subject by the recent 

 publication of the views of Dr. GL E. Blackhamf (also of Dr. J. 

 Edwards Smith ^), in favour of the retention of the correction- collar, 

 and he accordingly discusses the subject in some detail. 



If we consider the matter first from the side of theory, it must on 

 the one hand be allowed that the correction-collar, from a purely 

 theoretical point of view, may have certain (though as will be seen in 

 the sequel practically unimportant) advantages, while on the other 

 hand it is undoubtedly the fact that the other advantages ascribed to 

 it must be regarded as imaginary. » 



The advantages relate essentially to the following points : — First, 

 with the correction-collar we are not so strictly limited to an im- 

 mersion fluid of a particular index of refraction, as we are in the case 

 of the fixed mounting, but various fluids can be used, which are 

 different within certain — though always very narrow — limits. 



In the use of an immersion fluid not precisely of the same refractive 

 index as crown glass (which is the case with most of the immersion 

 fluids hitherto employed, except the thickened cedar-wood oil), we can 

 still obtain perfect correction for cover-glasses of varying thickness. 



Further, those aberrations (comparatively considerable) can be 

 corrected, which occur with dry preparations (rarely, however, coming 

 under consideration in the scientific use of homogeneous immersion), 

 if they do not adhere closely to the slide, but are separated from it 

 by a thin stratum of "air. 



Finally, the correction-collar allows the same objective to be used 

 with a longer or shorter tube, while otherwise one is confined to 

 somewhat narrow limits in the length. 



All the other advantages, however, urged by the advocates of the 

 correction-collar are only imaginary, such as the possibility of most 

 exact correction for the change in the index of refraction of a par- 



* Zeitschr. f. Instrumentenk., ii. (1882) pp. 269-74. 

 t Cf. this Journal, ante, p. 407. ;«<«*» 

 % 'How to See with the Microscope,' 1881. 



