SS of the Microscope. 257 
come the miserable definition caused by longer tube and deeper eye- 
iece. 
Itsoon became a canon, that a third eye-piece was quite high 
enough to give reliable results; and to such ANDREW Ross only 
appealed. 
Hence the most strenuous attempts were made to deepen the 
objective instead of purifying the older objectives of their unorthodox 
rays continually introduced by larger aperture or greater eccentrical 
refraction. 
At last we have, as it were, awoke out of a pleasant dream to 
find that the sole reason of the lower glasses such as the 1th and 3th 
not carrying the deep eye-pieces, such as a good telescope bears for 
instance, are the errors of the glasses rendered visible or exaggerated 
by the high magnifying power of the deepest eye-pieces. That is 
the very point admitted by all. It requires little logic to reverse 
the proposition,—That the eye-lenses would not, unless the defects 
existed in the objectives, give them such unpleasant development 
as to debar us from using as deep oculars as a good telescope. 
Let us recall our experience, and condense it into a sentence— 
The better the glass, the higher is the magnifying power it will 
bear. 
The new immersion {th (Powell’s) gives a finer definition with 
an E eye-piece than old-fashioned glasses of 1869 effected with a 
B ocular. In other words, the definition is about three times finer 
than before. 
No one doubts that the qualities of telescopes of the same focal 
length and aperture may be ascertained by the powers they will 
bear. No reasonable cause can be shown why similar conclusions 
should not be drawn from applying the same simple method to the 
microscope. 
Before explaining the double-star test it may not be out of place 
here to allude more in detail to the slow manner in which truth has 
been forcing itself upon us, and the microscope acquiring fresh 
powers. 
We are indebted to Dr. Wallich for first assigning a pyramidal 
form to the markings of diatoms. This wasa great leap from their 
mere lined character. A glass was once thought excellent which 
succeeded merely in displaying two sets of intersecting lines! We 
look back with wonder, now, to our blindness. At that time con- 
troversy ran high, as it does now, when each improvement in 
research is fiercely contested line by line, dot by dot! The Hippo- 
campus (Navicula) was once a ne plus ultra. Long years ago, 
with honest pride, Topping displayed the double lines of this 
Navicula. In 1841 the black Podura markings with a half 
inch were astounding. ‘Ten years later Messrs. Powell exhibited 
tT 2 
