J 80 Transactions of the Society. 



the object and the cover, must depend upon the refractive indices 

 of the thin glass and medium, upon the aggregate of refraction due 

 to their thickness, and the numerical aperture of the objective. 

 Practically the relation is considered to be exact, and the distance 

 between the objective series is made to equal the vertical distance 

 between the top of the thin glass and the surface of the object 

 during the process of correction. My impression, however, is that 

 the distances are not equal in fact, and that that between the 

 objective series is less than the vertical amount between the object 

 and the top of the glass when a proper correction has been made. 



It. has been put very forcibly by Dr. Dippel that if the shape 

 of the object is unknown, correction may be a mistake, and 

 that when the focus is at a lower plane than the summit of the 

 object, correction may positively mislead. 



There is no doubt that an image seen under a certain correction, 

 and which is stated to be normal, is modijQed by under- and over- 

 correction. 



How to get at the truth is difficult, except in the instance of 

 geometrical bodies and definitely parted lines, but the examination 

 of the same object by many observers with different instruments 

 gives experience, and without indulging in calculations, including 

 the method of least squares, it is finally settled that such and 

 such is the real shape. 



The possibility of error remains, however, and it is perfectly 

 evident that many a difi^erence of delineation of carefully investigated 

 objects results from non- correction and over- and under-correction. 

 One cannot but help thinking that the difficulties in correcting dry 

 objectives of high amplifying power and great numerical aperture, 

 will lead to the almost constant employment of immersion objectives. 

 And really the only researches which are rendered more difficult 

 by the immersion principle, are those which have rendered the 

 name of Dr. Dallinger so illustrious. There is no doubt that it is 

 impossible to prevent the admixture of the medium with the water 

 below the thin cover when minute organisms are followed here and 

 there and often close up to the edge of the glass cover. 



Amongst the results of not correcting objectives are want of 

 definition, haziness, and the production of certain colours, and this 

 last phenomenon is often observed in objectives which are corrected 

 up to a certain degree and fixed. It is the fashion to correct and 

 fix so as to obtain a certain amount of chromatic aberration, a ruby 

 tint being considered the best. This is to obviate the efiect which 

 the perfect achromatism of a glass of large numerical aperture has 

 on the eye. 



Nevertheless this everlastingly recurring tint is objectionable 

 and leads to misconception when dealing with extremely minute 

 artificially coloured objects. 



