175 



a sharp and brilliant definition, as seen with the immersion 

 lens and l-16th at the Quekett soiree of this year. 



Measurement of the aberration. 



The diameter of the mercurial drops having been carefully 

 measured, they must be selected in diameter according to the 

 power of the objective to be examined. 



The following appear convenient relations : 



Power . . . 200 400 800 IGOO diameters. 

 Objective . . . 1 in. ^ in. f in. ^ inch. 

 Diameter of globule . ^ ^ ^ yJj^ of an inch. 



Distance of bright discs one foot, separated by tAvo inches 

 interval. Adjustable stops being used the aberration will 

 be measured by the interval at which their reflected double 

 stars cannot be defined. As each particular globule varies 

 its curvature according to an unknown law of capillary attrac- 

 tion, it is absolutely necessary to select globules of deter- 

 minate size, and to place the discs at one and the same angle 

 of incidence upon the globule. Glasses supposed to perform 

 accurately reveal often egregious errors by the application 

 of this method of research. 



On the Visibility of the Beading of Diatoms and Lejpido- 

 pterous Scales. 



Nothing can be more vague than the estimation of size by 

 the unassisted sight ; and to avoid this vagueness a standard 

 is of essential service, notwithstanding the extreme variation 

 in ocular definition by different observers. The limiting 

 visual angle probably ranges from 1" to 6". A bullet-mark 

 upon a white target at 1000 yards represents, perhaps, the 

 smallest object visible to the best riflemen, which (if an inch 

 spot) very nearly represents 6". But for all observing 

 powers of the eye where comparison is required a much 

 larger visual range is absolutely necessary. 



On Ocular Potocr. — The beading of various objects, 

 such as the diatomacese and lepidopterous scales, affords 

 excellent means of testing the powers of the human eye. 



Let — ttattts be the diameter of a bead : P the masfnifyino: 



»xlOUO ' o J o 



power of the combined glasses ; the visual angle subtended 

 by an image of the bead, formed virtually at 10 inches' 



