12 
law of shadow being governed by the combined effects of 
aperture and refracting power, no trustworthy interpretation 
can be given of comparative appearances or of approximate 
form, and, consequently, the analysing effects of changing 
the angular aperture by means of such a contrivance as the 
aberrameter are in many delicate observations of considerable 
interest and value. 
§ LUI. In selecting Standards of Definition, known Objects are 
greatly preferable to the Conjectural. 
The development of the powers of the microscope have 
been mainly owing to the selection of natural objects as 
tests, chiefly of the striated character. But as quality is 
raised, striation recedes and beads appear in almost every 
test object of a striated character. The wavy Podura—that 
celebrated object—is no longer the finest test. The spines 
give way to beads. It would appear that such objects for the 
highest powers are a kind of mirage, deluding the observer. 
For forty years this object has been the chief guide to the 
opticians, and the chief standard by which to yalue the 
glasses and perfect their performance. 
Known objects are therefore to be preferred, and if a 
very perfect image can be formed of suitable objects of 
familiar properties, sufficiently minute and accurate to test 
the defects in question, observers may proceed with confi- 
dence in their researches. The watch-dial used for testing 
Lord Rosse’s leviathan mirror, as being a known object, acted 
perfectly. An object of a highly conjectural structure would 
have been of little avail for a refined testing. 
It is well known that the aberration rapidly diminishes at 
a conjugate focus as it moves up to the lens. If, therefore, 
an excellent objective be reversed, and an image of a distant 
object be formed in the focus of the microscope, the aberra- 
tion of this image will in a high degree be less than that 
produced at the longer conjugate focus, the aberration in 
general! being found to increase with the distance at which 
the image is formed from the objective, as in the case of 
lengthening the draw-tube of the microscope. Hence if a 
very fine objective be used to form a@ miniature of an object 
placed at ten inches’ distance, the aberration of a point in the 
image is very considerably less than when the objective is 
used in the ordinary way. 
Experiment 11.—A small watch-dial strongly illuminated 
i Subject to the special and fundamental compensations of the objective 
glass. 
