596 DE. G. W. EOYSTON-PIGOTT ON A SEAECHEE EOE APLANATIC IMAGES. 
prismatic colours, both within and without the sharpest focal point or image, forming 
concentric intersections, displaying coloured pencils passing either to or from their finest 
point of focal combination where colour should be destroyed (fig. 6). 
Experiment 3. — The disks (a) shown by the apertures T gf o °f an inch in diameter, and 
separated between centres fj, were now brought nearer to the objective O'. It was then 
observed that the image-disks (of above four times their proper size) began to separate ; 
and since the spurious disk retains its false annular expansion independent of the true 
magnitude, it became evident that the exact distance at which the test doublet was first 
divided, gave for other objectives a comparative measure of their aberration ; the very 
slight aberration in the image (of the sixteenth) being scarcely appreciable, especially 
when favoured by the advantage of the water-film to enhance the precision of definition 
on the immersion system. 
By such experiments, with the finest glasses obtainable, the existence of an aberration 
of material and measurable amount being thus established, the next question to be settled 
assumed the following character, viz. 
What was the nature of the aberration produced by displacement of the final focal 
image viewed by the eye-lens; and whether better effects could be produced by a different 
distribution of the magnifying-powers. 
It was now found that increasing the distance between the eye-lenses and the objective, 
gained power indeed, but caused the aberration to increase faster than the power gained. 
Intermediate Huyghenian eyepieces, inverted, were found to increase power but sacrifice 
definition ; the apparent aberrations seemed incorrigible, so that this plan was finally 
abandoned in 1864. Although by this means the Pleurosigma rhomboides was fairly 
shown to Messrs. Powell and Le aland, with their one-sixteenth objective (dated 1862), 
they stated this method had been tried long before and relinquished as useless to improve 
definition. 
Sliding-tubes (made by them at my request) were now furnished with a “ universal 
screw,” in order to admit a great variety of single and compound cemented lenses (more 
or less chromatically and spherically corrected) being inserted within the draw tube 
midway between the eyepiece and the objective. So, also, whole or parts of objectives 
were similarly applied, thus forming a microscope within a microscope admitting endless 
combinations of compensations. 
It now seemed perfectly clear that any attempt to improve high-power definition must 
be preceded by the attainment of a ready and decisive method of ascertaining whether 
the balance of compensations was equal, or, on the other hand, over- or undercorrected. 
The Image-test already described appeared to effect this object in the following manner. 
The finest glasses, it is well known, are constructed upon the principle of balancing 
