MICROSCOPY. 357 



A New Cover -adjustment for Microscopical Objectives. 



The well-known optician, Mr. E. Gundlach, has proposed a 

 new cover-adjustment for objectives, which, he claims, has 

 some great advantages over the old method. These advan- 

 tages, as he states them, are : (1) The adjustment exerts no 

 deleterious influence on the corrections of the aberrations, 

 and is equally as efficient for any thickness of the covering- 

 glass as for uncovered objects. (2) The working distance is 

 the same for any cover-thickness, except for immersion ob- 

 jectives. For this reason objectives of very short working 

 distance will, with tins adjustment, admit of even the thick- 

 est covering-glass. (3) The magnifying power is unchanged. 

 (4) The image is placed but slightly out of focus. (5) The 

 adjustment is very sensitive. (6) It can be made to indicate 

 exactly the thickness of the cover. All these advantages, 

 with others, are obtained, he claims, by discarding the old 

 method of adjustment by moving one or more of the systems 

 of lenses, and by placing a movable front of plane glass be- 

 fore the anterior lens of the objective, filling the space be- 

 tween with glycerin. A thinner or thicker stratum of this 

 glycerin, according to the distance between the plane glass 

 and the lens, gives the required adjustment {American Jour- 

 nal of Microscopy, June, 1878). 



Professor Abbe's Apertometer. 



This apparatus is intended to enable an exact measure- 

 ment of angular aperture of any object-glass, dry or immer- 

 sion, to be made ; and to afford a definition of aperture w T hich 

 is not limited by the maximum air-angle, which is independ- 

 ent of the medium in front of the lens, and which, at the 

 same time, by its theoretical signification, may give a direct 

 indication of the resolving power of an objective. It has 

 long been evident that the expression "angle of aperture" 

 is deceptive as an indication of resolving power, since this 

 is proportional, not to the angle itself, but to the sine of 

 the semi-angular aperture ; in other words, in large angles 

 the ratio of resolving power can bear no proportion to tho 

 mere number of degrees, the sines of such angles having very 

 small progressive increase. Professor Abbe proposes a new 

 name numerical aperture ; and by means of this "numeri- 



