372 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
two decades, and their influence upon the development of lenses, and 
the subsequent construction of the Microscope, it leaves very much to 
be desired. It is true that it may be fairly looked upon as a handbook ; 
but for this very reason, if only in the interests of the student, allusions 
might with some show of reason be expected to what has been done 
in other countries and by other workers than the always industrious 
Germans. 
The students contemplated by the author of this book are certainly 
those who may be supposed to use it thoroughly, and from whom, in 
many cases at least, may be expected not only a repetition of class work, 
but the prosecution of discovery and research. It may be quite right to 
explain what the immersion system of lenses is as concisely as is com- 
patible with practical usefulness to the user of the book ; this has been 
done ; but both water immersion and homogeneous immersion have a 
history of uncommon importance, and especially the latter imparts a 
distinct educational value to even its modes of employment. Certainly 
we may not charge the error of ignoring other workers than those of 
Germany on the practical deviser of the homogeneous system, for he has 
very frankly admitted the influence of one English worker in giving it 
origin. 
Again, in the discussion of the influence of the cover-glass as a means 
of introducing aberration, and the method of correcting it, we doubt 
whether the student is benefited by the omission of its history, any more 
than in similar circumstances an English, American, or French student 
would be benefited by a description of the apochromatic system of lenses, 
without reference to the laborious discovery of the Abbe-Schott glass 
which has done so much for the modern Microscope. 
This is even more apparent in the consideration of the value and 
application of that most powerful instrument in modern microscopy, 
the condenser — the only means by which the high qualities of modern 
lenses can be efficiently made manifest. 
Those w’ho have followed the history of the microscopy of the last 
twenty years know that even under the enlightened guidance of Abbe, 
although objectives and eye-pieces have been gradually brought to a 
higher and higher quality, and their corrections have been made more 
complete, yet in German centres the value, nay, indispensable importance 
of the substage condenser was not perceived. 
It may be frankly admitted that the broad scientific value of the 
Microscope was understood earlier, and more largely from the first, on 
the Continent than in England; but also from the very earliest times 
its highest possibilities have been most fully seen and brought out in 
this country. It is so to-day. The very highest critical images of the 
most delicate objects have been demonstrated by English, and mostly by 
amateur microscopy. But this has always been due to the persistent 
use of the substage condenser, an instrument which has been steadily 
progressing in optical qualities to meet the improvement in object- 
glasses and general optical systems. 
Without going back to the time of Tulley, it is enough to start in 
1850 with the “ Gillett condenser,” so generally and long used by 
English microscopists ; and from then until now' the leading opticians 
have applied their very best pow r ers to perfect the condenser as the one 
