338 
an equal magnifying power, and the aperture 
would admit of great extension without a pro- 
portional increase in the spherical and chro- 
matic aberrations. This suggestion has been 
carried into practice with complete success as 
regards the performance of lenses executed on 
this plan; but the difficulties of various-kinds 
in the way of their execution are such as to 
render them very expensive; and as they are 
not superior to the combination now to be de- 
scribed, they have latterly been quite super- 
seded by it. 
This combination was first proposed by Dr. 
Wollaston, and is known as his doublet. It 
consists of two plano-convex lenses, whose 
focal lengths are in the proportion of one to 
three, or nearly so, having their convex sides 
directed towards the eye, and the lens of 
shortest focal length nearest the object. In 
Dr. Wollaston’s original combination no stop 
was interposed, and the distance between the 
lenses was left to be determined by experiment 
in each case. A great improvement was subse- 
quently made, however, by the introduction 
of a stop between the lenses, and by the divi- 
sion of the power of the smaller lens between 
two; this is due to Mr. Holland.* By these 
means a combination may be produced, in 
which the errors are made to correct each other 
so nearly that all the advantages of a wide 
aperture with a very short focus may be gained. 
e general nature of the performance of a 
doublet or triplet may be understood from the 
adjoining figure, (fig.160,) in which LO L’ is 
Fig. 160. 
Diagram of the passage of rays through a doublet. 
the object, P a portion of the pupil, and D D 
the stop. The ager of light from the two 
extremities, L L’, of the object cross each 
other in the stop, and consequently pass 
through the two lenses on the opposite sides 
* Trans. of Soc. of Arts, vol. xlix. 
MICROSCOPE. 
of the axis O P; so that each becomes af- 
fected by opposite errors, which to a certain 
extent balance and correct one another. To 
take the pencil L, for instance, which enters 
the eye at RB, R B; it is bent to the right 
at the first lens, and to the left at the second; 
and as each refraction alters the direction of 
the blue rays more than of the red, and more- — 
over, as the blue rays fall nearer the margin 
of the second lens, where the increased power 
of the refraction, consequent upon the distance 
from the centre, compensates in some di 
for the greater focal length of the second lens, 
the blue and red rays will emerge very nearly 
parallel, and are therefore colourless to pede 
At the same time the spherical aberrati 
been diminished by the circumstance that the 
side of the pencil, which passes one lens 
nearest the axis, passes the other nearest the 
margin. This explanation applies only, how- 
ever, to the pencils near the extremities of the 
object. The central pencil, it is obvious, will 
pass through the same relative portions of the 
two lenses, and only an imperfect correction 
will therefore take place, and of those i 
from the intermediate points the eae 
correction will with their imity to 
er or to the ctcumferenee. "Hence dou- 
let is not a perfect magnifier; but it is 
much superior to a single lens, and may bees 
constructed as to show many of the usual test- 
objects,—especially those in which a moderate 
amount of penetration is sufficient, 
the definition be good,—in a very beautiful 
manner. Its angle of aperture, however, by 
which is meant the angle of the apex of the 
conical pencils of rays admitted by it, cannot 
be advantageously increased much 40° 
or 45°. But when the smaller lens is replaced 
by a combination of two others, so as to form 
a triplet, their joint aberration is so much less 
Fig. 161. 
Diagram to illustrate angle of aperture. 
A, lens with small opening, admitting onl peneila ; 
of rays diverting at pap of 15° " * 
with large opening, admitting pencils of 50°. : 
that it is more counterbalanced by the third — 
lens placed above the stop. In this manner 
the transmission of a still larger angular | 
a to 65°,—is ouial compati er 
istinctness; and great etrati i 
thus combined with perfect definition, "a 
well as with brilliancy of illumination. For 
the purposes of anatomical investi » as 
we shall hereafter state, we consider good 
doublets and triplets, where circumstances — 
admit of their employment, superior to any 
other kind of magnifying instrument. The 
principal disadvantages which the use of them — 
involves are the close poe’ to the object — 
required by their very short focus when a high 
