176 
decide whether compensations of aberration could be effected 
by attending to some definite principle or law. 
The previously ascertained properties of eidola 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 versd. 
II. With test-images, both observing and miniature or 
image-forming objective follow the same law of compensa- 
tion. If one be overcorrected the other must be similarly 
adjusted, and vice versd. 
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 overcorrected, as compared with a standard defining 
distance of 9 inches. 
Although a fine definition seemed now attainable by means 
of supplementary compensating lenses, if judiciously intro- 
ducing balancing compensations, yet their practical adjust- 
ments were innumerable and tediously accomplished. 
In the distribution of the power lenses, and in the applica- 
tion of a traversing searcher, it was indispensable that the 
object should be kept distinctly visible in the field of view, 
by a proper selection of lenses, whilst the optical compensa- 
tions were being adjusted. The form finally adopted is simply 
this :— 
A pair of slightly overcorrected achromatic lenses, admit- 
ting of further correction by a separating adjustment, are 
mounted midway between a low eyepiece and the objective, 
so as to admit of a traverse of two or three inches by means 
1 During 1865—1869 many experiments were tried with complete objec- 
tives 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 Géttingen 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, and ‘ Poggend. Annalen,’ 1869, 
vol. xvi, p. 467 (‘ Nature,’ Jan. 27, 1870). 
In the first he recommends an inverted Huyghenian 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. 
