138 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of observations with the Microscope are made upon objects under a cover- 
glass, it becomes an important subject of inquiry whether microscopic 
measurements are sensibly affected by the curvature of the covers. 
Espi cially is this true as to such experts as profess to be able to identify 
blood by the measurement of the red corpuscles. I have not been able 
to find in the literature any investigation of this subject, and having in 
the Cronin case been called as an expert with reference to the identifica- 
tion of blood by micrometric measurement of the red corpuscles, I made 
the observations herein recorded, not with a view to publication, but in 
order to be able to come to a correct conclusion as to one factor in 
that case. 
In the comparison of long standards this question has no importance 
whatever, inasmuch as they are always uncovered ; but inasmuch as most 
objectives in common use are corrected for covered objects, or are usually 
so used, the value of one division of the micrometer used with such 
objectives on covered objects must be determined from a micrometer 
having its lines covered either permanently or temporarily. 
It is obvious that sensible curvature of a cover-glass interposed 
between the object and front of the objective must affect the ampli- 
fication of the objective. If such curvature is spherical this effect will 
be symmetrical ; if the curvature is irregular it must distort the object, 
so that the interval between two objects will appear to be different 
according to the orientation of the cover-glass. 
Several methods are available for testing the curvature. It may be 
tested by viewing with a telescope the image of some regular object, 
such as a building reflected from the cover-glass. It may also be tested 
by accurately focusing a telescope upon some object, such as an artificial 
star, and then interposing the cover-glass to be tested between the 
object viewed and the objective of the telescope. If the cover is flat and if 
its sides are parallel, such interposition will have no effect upon the 
image other than very slightly to diminish the amount of light received 
from it. If the cover-glass is spherical it will change the focus of the 
telescope, which must be refocused in order to obtain a clear image 
of the object observed. If the cover-glass has irregular curves the 
image will also be distorted. If the cover-glass is prismatic the image 
will be displaced laterally. 
A very much more sensitive method than either of the above is that 
by the observation of the Newton’s rings, caused by placing the cover- 
glass upon a flat surface. For this purpose I obtained one of Mr. 
Brashear’s justly celebrated flat test surfaces ; but having found upon 
trial that the second method above described was sufficiently sensitive 
for my purpose, it was used exclusively in the observations hereinafter 
described. Tested by this method, I may state that # nearly all the 
cover-glasses in common use possess either regular or irregular curva- 
ture. Tested by the method of Newton’s rings, I am informed by 
Mr. Brashear that it is practically impossible to find one perfectly flat. 
I have myself examined and tested a large number furnished me by the 
courtesy of the Bausch & Lomb Optical Company and the Palmer Slide 
Company, as well as some purchased of Mr. Zeiss of Jena, and have 
found a very few that by the second method above described were sen- 
sibly flat. By far the greater number possessed quite sensible curvature. 
