Testing Ohject-glasses. By Dr. Boysion-Figott. 153 
the obliquity at wliicli the miniature is formed in the field of view 
by inclining the sub-stage. The surpassing beauty and perfec- 
tion of these figures thus obtained by the magnified miniature of 
the mercurial solar star, render it probable that the obliquely 
illumined mercurial globule, viewed directly in close proximity to 
the front of the object-glass of the microscope, and placed upon the 
stage, is a very imperfect test; and the methods here described 
are submitted as possessing very superior delicacy and convenience. 
The perfection of the diffraction lines cannot be displayed at all 
by the old method. 
Eidola. With such instances of wonderful variation of the 
spectra formed by the miniature of a sun-lit mercurial globule, we 
may well suspect that innumerable difiraction images may be 
developed : 
(1) By obliquity of illumination. 
(2) By erroneous correction of the glasses. 
(3) By erroneous focussing. 
I. An absolute knowledge of structure cannot be probably ob- 
tained by obliquity of illumination. Those structures which can 
only be thus seen are liable to distortion and misrepresentation. 
Example (1). Display very fine gauze by oblique light and a 
low power a little out of focus, a complex structure is seen bearing 
very little resemblance to the reality. 
Example (2). Treat the transparently-mounted eyes of an 
insect in the same way. The eidolic forms of delusion are endless. 
The diffraction effects are the most exaggerated when the illumina- 
tion is most oblique. 
II. Example (1). Illuminate perforated metal from behind in 
a darkened room ; view an exquisite miniature of these perfora- 
tions. The holes will appear enlarged ; black dots take the place 
of apertures ; haloes join haloes, and the images are altogether 
disguised and transformed according to the correction of the 
observing objective and the focal plane of vision employed. 
Example (2). Fine copper wire gauze thus treated loses its 
apparent solidity. The meshes appear chequered with black dots, 
sharply defined, and the wires appear translucent. Thus in an 
unknown structure these false eidola might readily be mistaken for 
the true images. In this way a variety of forms having no reality 
start occasibnally into almost tangible form and existence. 
III. Erroneous focussing. 
Microscopists are not always agreed in viewing an unknown 
object which is the correct focal plane. And it is just possible 
that different observers with the same instrument and adjustment 
view a different focal picture. 
