1228 REVIEWS. 
for the information of beginners that the lenses previously 
mentioned are, by the same accepted standard, far less than sec- 
ond quality. The one-inch lenses furnished lately with Tolles’ 
students’ microscopes have been too cheap to be good; but his 
second-quality one-fourth, one-sixth, ete. (which are most used by 
physicians), as well as similar powers by Zentmayer, Grunow, etc., 
are incomparably ial than the cheap lenses, often called trip- 
lets, mentioned aboy 
It is stated that oe ieii objectives are each composed of three 
pairs of lenses or doublets, properly arranged ; although it is un- 
derstood by microscopists that the posterior combination is often 
a triplet, and that the anterior pair is often replaced by a triplet 
or BY a single lens. 
In regard to the qualities of the objective there is an unfortu- 
nate want of clearness and sharpness in the author’s use of terms; 
‘penetrating power being used sometimes in reference to that re- 
solving power which is the joint result of definition and angular 
aperture, and sometimes as descriptive of that depth of field to 
which its use seems to be generally and usefully confined. 
But the most astounding assertion in regard to’ objectives is 
this, that the lenses of various opticians are almost equally suc- 
cessful up to powers as high as one-eighth inch! If "those opti- 
cians, whose names are familiar words, have achieved eminence 
anywhere it has been in the eacellence of their low and medium 
powers. Tolles’ most remarkable lenses are probably from one- 
fourth to one-eighth inch focus; and Wales’ four-teriths (one-. 
fourths?) and one-fifths are nearly equally famous powers. Ross’s 
lenses, of which none are of very high power, have enjoyed a con- 
ceded preéminence ; Smith and Beck’s four-tenths has been classi- 
cal for years, while their high power, one-twentieth, has been of 
very moderate use ; and Powell and Lealand’s one-eighth and one- 
-sixteenth have doubtless contributed more renown than their one- 
twenty-fifth and one-fiftieth. And it is probably safe to predict, 
as the opinion of most microscopists, that the progress of the near 
future belongs to those opticians who shall use the most extraor- 
dinary skill in perfecting and utilizing lenses of moderate focal 
length. 
The subject of test-objects is treated at some length for the 
‘avowed benefit of such students as might find it difficult to secure 
the assistance of an expert in selecting their lenses. Doubtless 
it would have been safer to advise the student in such cases to 
