262 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
tlie brightness of the different points of the source is different : for every 
part of the plane of the object is illuminated by a pencil to which every 
single point of the source contributes rays. 
A disadvantage to this method of illumination is that the source of 
light comes too close to the object. Other difficulties are that a gradua- 
tion of the angular aperture of the illuminating pencil cannot be conve- 
niently obtained, and that a sharp limitation of the illuminated surface 
in the plane of the object is impossible. 
The author succeeds in obviating these three difficulties by the 
following devices. 
The heating of the object due to the close proximity of the source of 
light is avoided by not bringing the source itself into the hinder focal 
plane of the condenser, but rather a magnified or diminished image of it 
produced by a suitable condensing lens of not too small focal length. 
The second difficulty is then also removed by the use of suitable dia- 
phragms in the plane of this image. 
The third difficulty is got rid of by the use of what the author calls 
a field of view diaphragm ( Sehfeldblende ) placed in the conjugate plane 
of the condenser to the plane of the object. The condenser then throws 
a real image of this diaphragm in the plane of the object, the size and 
form of which depend upon the size and form of the diaphragm. 
The whole path of the rays is seen in fig. 23, taking into account now 
also the part below the line A B. The source of light is no longer in 
this plane, but far below at A 1 A 2 , and L x L 2 now represents its image 
produced by the condensing lens s l. The “ Sehfeldblende ” is at s b 
and oj co 2 represent conjugate points to o 1 o o 2 . 
The author adds some practical suggestions as to the adjustment of 
the apparatus, and choice of lenses and sources of light. 
Stereoscopic Photomicrography.* — Dr. W. C. Borden points out the 
advantages offered by stereoscopic photomicrographs in representing 
microscopic objects in their real form through the effect of relief. The 
advantage generally claimed for a drawing over a photomicrograph is 
that the former may be constructed so as to represent an object as it 
appears viewed at different focal planes ; but by means of stereoscopic 
photomicrography a similar result may be obtained, for the two most 
important planes of an object may be superimposed and combined so that 
a picture having natural relief and sharpness is produced. This result 
is attained by using a different focus for each exposure in the two nega- 
tives necessary for the stereoscopic picture. When viewed in the 
stereoscope the sharp outlines of each negative are found to override the 
blurred outlines of the other, and a single stereoscopic picture is produced 
presenting different planes of the object in sharpness and relief. 
In taking stereoscopic photomicrographs the necessary lateral views 
of the object are obtained either by tilting the object or by using different 
halves of the objective for each view. With low power objectives the 
first method is the best to employ. In tilting the slide the axis of the 
tilt must be parallel with the upright axis of the object. 
Remarkable Collection of Photomicrographs.! — Mr. K. M. Cun- 
ningham calls attention to a remarkable collection of photomicro- 
* Amer. Micr. Journ., xiv. (189?) pp. 329-33. f Tom. cit., pp. 339-42. 
