NOTES AND MEMORANDA. 
253 
The advantages of this arrangement are obvious, and summarized 
and recapitulated are as follows : — 1. The adjustment exerts no 
deleterious influence on the corrections of the aberrations, and is 
equally as efficient for any thickness of the covering glass as for un- 
covered objects. 2. The working distance is the same for any cover 
thickness except for immersion objectives ; for this reason objectives 
of very short working distance will with this adjustment admit of even 
the thickest covering glass. 3. The magnifying power is unchanged. 
4. The image is placed but slightly out of focus. 5. The adjustment 
is very sensitive, thereby facilitating the exact rectification. 6. It 
can very easily be so arranged that the graduation on the adjustment 
rim shall indicate exactly the thickness of the cover. 7. Any casual 
and unavoidable defect in the movement of the adjustment has no in- 
fluence on the centering of the objective, as any lateral displacement 
of the parallel disk causes no optical change whatever. The fluid 
between the front lens and the transparent parallel disk is glycerine, 
which has so far answered all expectation in a most satisfactory 
manner. Objectives made seven months ago have been sent long 
distances by rail without impairing their efficiency, and without loss or 
renewal of the glycerine. They might perhaps be used for years 
without the presence of any fluid being suspected. The new adjust- 
ment is applicable as well to immersion as to dry working objectives. 
It deserves to be mentioned, however, that for immersion objectives 
the advantages of equal working distances for every cover-glass thick- 
ness do not exist. It is, however, the reverse of what it is with the 
old adjustment, as the working distance will be the further the thicker 
the covering glass happens to be.* 
The American Microscopical Congress. — This Congress — it is 
believed the first of its kind — met at Indianapolis, Ind., on the 
14th-17th August last, under the presidency of Dr. E. H. Ward, of 
Troy. The following papers were read : — 
Aug. l^th . — “ On the Limits of Accuracy in Measurement with 
the Microscope,” by Prof. W. A. Kogers. “ Some New Forms of 
Mounting,” by C. C. Merriman. 
15th . — “ On Mechanical Fingers,” by C. M. Vorce. “ On Angular 
Aperture of Microscope Objectives,” by Dr. Geo. E. Blacliam. 
“ On the Definition of the term Angular Aperture,” by Prof. E. 
Hitchcock. “ On the Preparation of Ashes of Leaves for the Study 
of Structure,” by Dr. E. H. Ward. “ On the Classification of Algae,” 
by Eev. A. B. Hervey. 
16//^. — “ On the Emigration of Blood -corpuscles in Passive 
Hypereemia,” by Dr. W. T. Belfield. “ On a Standard Micrometer,” 
by Prof. E. Hitchcock. “ On a handy Eule for Micrometry,” by 
C. M. Vorce. “ On the Sting of the Honey Bee,” by J. D. 
Hyatt. 
17th . — “ On a New Section Cutter,” by Dr. Carl Seiler. “ On 
Biscol’s Section Cutter,” by Dr. E. H. Ward. “On the Progress of 
Microscopic Euling,” by Prof. J. Edwards Smith. “ On the Construc- 
* ‘ American Journal of Microscopy, vol. iii. 135. 
