1877.] Microscopy. 633 
ble test, as the aberrations of the objective, particularly the distortion, 
are easily confounded with those of the eye-piece. — E. GUNDLACH. 
OBJECTIVES AS ILLumINaTORS.— Mr. George W. Morehouse urges 
strongly the use of the best attainable objectives as substitutes for the 
various illuminating accessories furnished with microscopes, on the prin- 
ciple that the illuminator should exactly equal the magnifier in capacity 
in all respects, which he has not found true of any of the illuminators. 
By using immersion objectives of the highest angle, properly centred and 
focused, as achromatic condensers, he believes the truest appearance of 
the object is obtained, and with the least liability to errors of interpreta- 
tion, the images of structure lying in different focal planes being sep- 
arated with the greatest accuracy. Of course, the objects for this use 
must be mounted in balsam between two thin cover-glasses, or if 
mounted dry they must be in actual contact with both glasses. 
He has also had more than ordinary success with the plan of making 
the objective its own illuminator for opaque objects, first successfully in- 
troduced by Prof. H. L. Smith. He uses the form known as Beck’s 
Vertical illuminator, which is a thin glass disk in an adapter above the 
objective, light from a flat-wicked lamp placed edgeways, at a distance 
of about eight inches, being reflected by the disk through the objective 
upon the object. The image of the flame should be seen in the centre of 
the field, where, with immersion objectives of the highest angle, details 
of surface structure can be seen with the greatest distinctness with powers 
as high as four thousand diameters. The projecting spines upon the test 
podura scales can be thus seen both on the surface and at the edge of the 
scales; and even objects that only imperfectly reflect the light, such as 
diatoms, can be distinguished with clearness and beauty. Pleurosigma 
um is seen in hexagons, and A. pellucida shows the striæ sharply 
When the illumination is rendered one-sided by the hand or any other 
obstruction held partly between the flame and the reflecting disk. 
MOUNTING IN DAMMAR. — The occasional failure of specimens mount- 
ed in dammar to keep well, by reason of the external ring of varnish run- 
hing in, or other disaster due to imperfect hardening of the dammar, has 
ed to an interesting and useful discussion in the pages of Science Gossip. 
The best method seems to be to place the object, previously soaked in 
turpentine or benzole, on the slide, and then either drop the dammar on 
the object and press down on it the slightly warmed cover-glass, or else 
place the object from the turpentine on the slide, cover it with the cover- 
S'ass, and allow the dammar to flow in assisted by a moderate warmth, 
~~ in either case the slide being at once transferred to a metal plate about 
six inches above the flame of a spirit lamp. As the heat should not be 
Sufficient to boil the dammar, air bubbles will not form, and the progress 
s the drying need not be closely watched while attending to other work. 
about an hour the dammar will be so hardened as to be quite safe, the 
Precise time to remove it from the plate being determined by taking up 
