162 ON THE CHOICE OF A MICROSCOPE. 
it occupies so prominent a place in the structure of the 
microscope, but also because the preceding discoveries, 
though many years prior to the eight recent improvements 
spoken of in the earlier part of this paper, are of by far the 
greatest importance. And now, before making any further 
remark, it will be necessary to give some explanation of the 
four terms used to denote the qualities of an object-glass ; 
namely, Resolution, Penetration, Definition, and Flatness of 
field. 1st. The term Resolution is used to signify the power 
of an object-glass to show clearly the minute details on the 
surface of objects such as dots or lines, and which is effected, 
not so much by increase of magnifying power, as by increase 
of light transmitted through the object-glass to the eye by 
the enlargement of its angular aperture. 2nd. The term 
Penetration, according to its modern acceptation, denotes that 
quality of an object-glass which enables the observer to see deep 
into the structure of objects, not merely any delicate mark- 
ings on their surface, but what is below them as well, without 
any alteration of focus; to show with perfect distinctness 
such parts as are in focus, and with tolerable clearness those 
parts that are a little out of focus besides. 3rd. The term 
Definition is used to signify the capabilities of an object-glass 
for showing the various details of an object, especially its 
boundaries, with the most perfect and exquisite sharpness. 
4th. The expression marginal and central definition, in other 
words, Flatness of field, is used to denote the capacity of an 
object-glass for showing the plane surface of an object as 
sharply defined at the margin of the field of view as it is in 
its centre. It would be well for a person about to buy a 
microscope to have these four terms and their definition 
clearly in his mind—at his fingers’ ends, so to say; because 
it is on the union of the above qualities in the highest degree, 
as far as they are consistent one with another, that the good- 
ness of an object-glass mainly depends. Anyhow, I trust the 
preceding explanation will help to clear up the meaning of 
the very few remarks which I proceed now to make. Dr. 
Carpenter in his work on the microscope observes that some’ 
opticians, in their zeal to increase the resolving power of 
object-glasses by the excessive enlargement of their angular 
aperture, unduly sacrifice the still more important qualities 
being a doublet composed of a double convex lens of crown, with a double 
concave of flint glass; and the front combination being another triplet com- 
posed of two plano-convex lenses of crown, with a plano-concave lens of 
flint between them. ‘This will explain the composition of the shallow object- 
glasses, the distance between the different combinations throughout all the 
objectives and their size depending on the amount of magnifying power 
required. 
