260 
CORBESPONDENCE. 
The above objectives are for various objects : the moisture has 
very little tendency to collect in them ; but it is not the same in the 
immersion working in various fluids. Also, would it not be an advan- 
tage to get over this inconvenience by facilitating the cleaning of the 
optical arrangement without unscrewing the collar ? 
One of our systems is based upon the correction by Wenham, 
adopted by Nachet and others ; and the other is based on the move- 
ment alone of the last lens, or second corrective. The first is more 
handy for quickness of adjustment, and the second offers greater 
facility for the centering of the optical arrangement. 
Fig. 1 represents my objective corrected by Wenham. A cone 
fixed to the interior tube E E E E, into which are screwed the 
corrective lenses A B, A B. The collar E F advances or withdraws 
the front lens H from the two correctives. The front lens H is fixed 
in a ring, screwed at 1 1 to the lower extremity of the exterior tube 
Q Q Q Q, in the part G G of which is screwed the tube of the front 
lens H. The correctives A A, B B screw into the cone of the interior 
tube E E E E. When it is necessary to clean these, this tube must 
be unscrewed as the piece G I, G I, then the two correctives A A, B B 
from the cone C D. To clean the front lens H, the ring of the tube 
G I, G I, in the centre of which it is fixed, must be unscrewed. This 
operation does not necessitate any undoing of the collar E F. 
Fig. 2 represents my new objective, with movement by the second 
corrective. B B and D D represent the mounting of the front lens C, 
and of the first corrective B B ; but the second corrective is mounted 
in a screwed tube A S, A S, put in motion by the collar E R. The 
optical arrangement is easily cleaned, without undoing the collar E E, 
by first unscrewing the two lenses, placed under the second corrective 
A A, the front and the first corrective. 
This system appears to us more simple than the Wenham system. 
Your obedient servant, 
Clement Thuet. 
[We insert the above as possibly containing an idea of practical 
value, but it is so obscurely described that we fear our readers will 
scarcely be able to comprehend it. — Ed. ' M. M. J.'] 
Professor Hasert's New Objective. 
To the Editor of the ' Monthly Microscopical Journal.' 
Denstone, Uttoxetek, October 3, 1875. 
Sir, — Professor Hasert of Eisenach, who, as the readers of this 
Journal will probably recollect, manufactured the lenses with which 
Dr. Schumann performed those truly wondrous feats recorded in his 
'Diatomcen der hohen Tatra,' has been for a considerable time 
engaged in devising an altogether novel species of lens, which will, I 
expect, form an epoch in microscopy. The problem he proposed to 
