Journal aSTiS ] Ohject-ghsses for the Microscojpe. 225 
YII. — On the Construction of Object-glasses for the Microscope. 
By F. H. Wenham. 
{Continued from page 173, No. III.) 
The lens to be tested is adapted to the microscope, having the 
ordinary Hnyghenian eye-piece. On placing the globule either in 
or out of focus, the luminous point expands into a ring. If the 
object-glass is under-corrected for colour, as in a single lens, the 
bright ring appears within the focus, the outer margin is red, and 
the inner circle green. If the lens is over-corrected, the bright 
ring appears without the focus, with the colours in the same order 
as before. A practical knowledge only, derived from these appear- 
ances, can determine the amount of concavity to be given to the 
flint, or difference of convexity in the crown, for obtaining the 
desired correction ; but even in the most experienced hands it gene- 
rally involves several alterations to secure perfect achromatism. 
When this is corrected as far as practicable, a pale-green colour only 
is perceptible beyond the focus. This arises from the secondary 
spectrum, or relative difference in the width of the prismatic colour 
spaces of the crown and flint, and seems to be a variable condition, 
according to the composition of the glass employed. 
Though correction for spherical aberration is intimately related 
to that of colour — a single lens, when finally achromatized, being 
also nearly free from spherical error ; yet, in a combination of three 
pair, when matched so as to be achromatic, this may be so consi- 
derable as to render the object-glass useless, and is oftentimes 
exceedingly troublesome to remedy. The error may arise from an 
improper proportion between the relative foci of the lenses — as the 
back being too long. I have before stated that in the form that I 
have advocated, the spherical aberration is mainly corrected by 
giving thickness to the front lens, and by properly adjusting the 
distance between them. In a glass spherically under-corrected 
the Hght from the globule is greatest within the focus, and when set 
out of focus speedily vanishes and becomes diffused ; in the case of 
spherical over-correction the contrary appearances result. When 
the relative distance of the lenses is rightly adjusted, the light spot 
expands equally, and is of the same intensity, for a short distance 
on either side of the focus, in which the globule should appear with 
a clear bright margin. The object-glass is now in a proper condi- 
tion for testing errors of construction and workmanship. 
To examine the condition of the oblique pencils, and consequent 
flatness and distinctness throughout the field, a small globule is 
selected, and brought to the edge, using the lowest eye-piece ; if the 
bright point in the centre of the globule, when a Httle out of focus, 
