112 Construction of Object-glasses \^3oSl\^^^^T^lt 
under-corrected for that purpose. But on attempting to improve 
the correction by a difference in the radius of the concave flint of the 
triple front, it was shown that a considerable alteration was here re- 
quired to effect a material correction for colour. Taking a ray at 
the focal distance from the front surface, and tracing its refraction 
through the triple, at all points it appeared to enter the concave 
surface nearly as a radius from its centre. Consequently, under 
this condition, the effect of the dense flint was partly neutralized. 
It then occurred to me to try a single lens for a front. With 
this combination no satisfactory result could be obtained with respect 
to achromatism. 
Early in the year 1850, Mr. Lister was occupied with experi- 
ments for the purpose of improving the higher powers, and then 
introduced the triple hach, which has since so eminently proved to 
be the grandest step towards their perfection, allowing perfect 
correction to be obtained with the most extreme apertures. 
Having received early information of this improvement, I set to 
work and again tried the single front in combination with the triple 
back, and constructed an J-th on this system which is considered 
excellent to this day. For several years I stood alone in my opinion 
of its advantages ; but as numbers of our best object-glasses of the 
highest powers are now made v^ith single fronts, I am in a better 
position for advocating this form, particularly as its success was 
found to depend upon a relative difference of focal lengths in the 
two back combinations not hitherto employed by others. 
At first the single front with the back triple was not successful. 
Though colour was nearly corrected, there was a deficiency of 
aperture, and the combination was spherically under-corrected. On 
viewing another object under a thicker covering-glass, the definition 
was greatly improved. By placing other pieces of thin glass over 
the object, the front lens had to bo 
drawn still closer to the others. 
This gave an increase of aperture 
and more perfect definition. A 
single front was then made, of the 
thickness which had been found to 
give the best result, ascertained 
from the measurements of the addi- 
tional pieces of thin glass over the 
object, and the effect was all that 
could be desired. On finding that the 
correction for spherical aberration 
depended upon the thichiess of the 
front lens, the path became easy. 
The cut represents an ^th of 130^ of aperture constructed 
on this system, six times the size of the original. The curves 
