DR. G. W. ROYSTON-PIGOTT ON A SEARCHER EOR APLANATIC IMAGES. 599 
whether compensations of aberration could be effected by attending to some definite 
principle or law. 
The previously ascertained properties of eulola enabled many experiments to be made 
with rapidity and certainty. The following principles were, in short, patiently arrived 
at by experiments extending over several years : — 
I. Displacement of the final focal image towards the eye-lenses, provided the front 
lens or facet of the object-glass is kept at the same distance from the object under 
observation, is caused by approximating even slightly the component adjusting lenses of 
the objective, and this movement causes a negative aberration, and vice versa. 
II. With test-images, both observing and miniature or image-forming objective follow 
the same law of compensation. If one be overcorrected the other must be similarly 
adjusted, and vice versa. 
III. Using additional compensating lenses to gain increase of power, intermediately 
placed between eyepiece and objective, the finest definition is obtained when each of the 
three sets, viz. lenses, observing and image-objective, are similarly though slightly over- 
corrected, as compared with a standard defining distance of 9 inches. 
Although a fine definition seemed now attainable by means of supplementary com- 
pensating lenses, if judiciously introducing balancing compensations, yet their practical 
adjustments were innumerable and tediously accomplished* *. At this stage of the re- 
search, frequent consideration of the well-known optical equations for a vanishing aberra- 
tion fortunately suggested to me the idea of searching the axis, mechanically, for apla- 
natic foci. In reference to these equations, which would be out of place here, it has 
been observed by Dr. Parkinson, F.R.S.f, “If the aberration for rays parallel at inci- 
dence of a compound lens of given focal length — consisting of several thin lenses in con- 
tact — be examined, it will consist of a series of terms similar to that in Art. (129), one 
term for each lens, and the condition that the aberration shall vanish will lead to an 
equation involving more than one unknown quantity, and consequently admitting an 
unlimited number of solutions.” 
In the distribution of the power-lenses, and in the application of a traversing 
searcher, it was indispensable that the object should be kept distinctly visible in the 
objectives remain unknown. As one of the Jurors in the Paris Exposition, his microscope necessarily remained 
both uncelebrated and unelucidated in the Reports. 
* During 1865-1869 many experiments were tried with complete objectives and various parts of them, either 
over- or undercorrected by means of a sliding-tube carrying them and fitting into the “ draw tube.” 
Professor Listing of Gottingen has confirmed the value of this method of amplification quite independently in 
two papers published in 1869. Nachr. d. kgl. Gesell. der Wissensch. 1869, No. 1, andPog-gend. Annalen, 1869, 
vol. xvi. p. 467 (‘Nature,’ Jan. 27, 1870). 
In the first he recommended an inverted Iluyghenian eyepiece, and in the second intermediate achromatic 
lenses. 
As regards intermediate lenses, the writer has ascertained (Nov. 1870) that Dr. Goring (Micrographia, ed. 
1837) has anticipated both these methods. — Note added Nov. 1870. 
f Griffin’s ‘ Optics,’ by Parkinson, p. 122, 2nd ed. 1866. 
MDCCCLXX. 4 N 
