THE YOUNG SCIENTIST. 
121 
aberration or over-correction could readily 
be commanded with lenses of shallow, 
contact curves. During these trials all 
chromatic correction was obtained by 
alterations in the triple back ; for it was 
found that the color-correction could not 
be controlled by a change in the concave 
surface-of the triple front, as the negative 
power of the flint here appeared to be 
feeble, requiring a great difference in 
I radius to give a trifling result. For this 
I reason the front concaves were formed of 
I very dense and highly dispersive flint ; the 
; cause of this was analysed by a large 
diagram, with the passage of the rays 
projected through the combination, start- 
ing from the longest conjugate focus at 
the back. This proved that the rays from 
that focus passed through the concave 
flint of the front nearly as a radius from 
its centre, or in such a direction that its 
negative influence was almost neutralised. 
It is well known that a lens may be 
achromatic for parallel rays, and under- 
corrected for divergent ones. The utmost 
extent of this condition was apparent in 
the object-glass under consideration. 
This led the author to the idea of the 
single front lens of crown-glass, which 
gave a fine result at the first attempt, as 
the back combinations to which it was ap- 
plied happened to have a suitable excess 
of negative or over-correction existing in 
the triple back alone, the middle being 
neutral or nearly achromatic. Still there 
was a defect remaining as positive spheri- 
cal aberration ; and this was afterwards 
cured by giving additional thickness to the 
front lens, which is now recognized as a 
most essential element of correction. In a 
"fifteenth," for instance, a difference of 
thickness of only '002 of an inch will de- : 
termine the quality between a good and | 
indifferent glass. Fig. 17 represents a ; 
front lens suitable for bringing the back i 
rays to a focus. The dotted lines indicate j 
the effect of this difference, showing that 
with a lens of less thickness the marginal 
rays fall within the central, producing 
positive aberration as the result. 
The single front introduced by the 
author (Mr. Wenham) is now used by 
every maker : for several years he could 
not induce opticians to change their 
system, though challenged by a series of 
high powers constructed on this formula 
for the purpose of proving its superiority. 
Fig. 16 represents the curves of the 
first successful " eighth " on this system, 
having an aperture of 130°, enlarged ten 
times. On tracing the passage of the 
marginal rays through the combination, 
I it will be seen that, though the successive 
1 refractions are nearly equalized, the con- 
1 tact-surfaces of the middle pair are some- 
I what deep, though no over-correction 
existed or was needed here, for this would 
! have required a shorter radius still (the 
i density of the flint in this w^as 3*686). If 
I this pair of lenses was not cemented with 
1 Canada balsam, total reflection would 
I take place near the circumference of the 
I contact flint surface, cutting off' the 
I marginal rays at a, and limiting the aper- 
I ture. It might be argued that practically 
I this would be no disadvantage, as these 
1 surfaces are united with Canada balsam, 
1 whose refraction is higher than the crown ; 
so that the rays in this case must proceed 
with very little deviation. But incidences 
beyond the angle of total reflection may 
be considered detrimental, as they imply 
excessive depth of curvature ; this can be 
discovered by looking through the front of 
an object-glass held close to the eye, any 
air-fllms in the balsam near the edge of 
the lens appearing as opaque black s,i>ots. 
Science in Common-Place Things— The 
Syphon. 
IN our previous issue we stated that the 
author of " Physics Without Appara- 
tus " not only used special ai^paratus in 
his experiments, but that in some cases he 
employed apparatus which could be pro- 
cured only from dealers in philosophical 
apparatus. The engraving which we give 
this month, representing the well-kno-wn 
Cup of Tantalus, fully sustains this posi- 
tion. The Cup of Tantalus is, however, 
an illustration of such a useful instru- 
ment, that a short description of it will 
prove interesting and useful. 
Many of our readers have probably read 
the story of Tantalus, whose name is em- 
bodied in our language in the word tanta- 
lize. Various accounts are given of the 
