22 
MICROSCOPICAL SECTION. 
“Methods of Photo-micrography,” by Mr. A. E. GOODMAN. 

By means of lantern slides the many makes and styles of both British and 
Continental Photo-micrographic apparatus were rapidly reviewed, and later, 
the various parts, and their uses, of a modern microscope were described. 
It was shown that, whilst all the mechanical contrivances and optical 
arrangements associated with the complicated microscope of to-day were 
most helpful, when properly manipulated, to the microscopist it was the 
objective, after all, which was the one part above all others, which needed 
the greatest care in selection; for upon the objective depended the amount 
of detail in any given object which would be visible through the microscope. 
The defining power of a given lens was shown to depend upon the 
number of “‘ spectral images,”’ or ‘‘ diffraction spectra,” formed by the object 
under examination, and admitted or collected by the objective and passed 
into the microscope tube. For the purpose of illustrating the nature of these 
spectral images, a number of gratings of very fine and closely placed lines 
produced photographically upon glass had been prepared. Each grating was 
of a double nature, similar to the one shown in Fig. 1, but more extended, 
and the lines very considerably closer and finer. 
On viewing a point of light through such a grating, the source of light 
appeared as a central beam bounded on either side by a regularly spaced 
series of diffraction images which decreased in brightness as they receded 
from the central beam, and were ¢wzce as numerous when viewed through the 
wide, as when viewed through the marrow spaces. The positions of the 
spectral images and central beam when viewed through such a grating are 
indicated in Fig. 2. This appearance or formation of diffraction spectra 
occurs in the same manner when working with the microscope ; and in order 
to further explain the working of an objective some of Dr. Abbé’s experi- 
ments were described in detail. A scale similar to Fig. 1 is prepared on a 
glass slip, the coarser lines being about 1,500 to the inch; when this is 
placed in the field of a suitable objective and focussed, a double series of 
diffraction spectra are produced, as shown in Fig. 2, and which may be 
seen on removing the eyepiece of the microscope and looking down the tube. 
By placing above the back lens of the objective discs of metal pierced by 
various slits, any set of these spectra may be allowed to pass up the tube, 
and the others shut off. In this way the upper series of diffraction spectra 
may be made to resemble the lower series in Fig. 2. When the light 
admitted by such a disc (Fig. 3 A) is examined through the eyepiece, the 
appearance presented is that shown in Fig. 3 B, where the grating appears 
to consist throughout of narrow lines—the lines in the upper part of scale 
having been doubled by shutting off the alternate diffraction spectra. 
By arranging the slits in another disc (Fig. 4 A) further apart, so that the 
fourth spectral images on each side of the upper series and the corresponding 
second images of the lower series only are admitted, along with the central 
beams, the whole of the lines are again doubled in number (Fig. 4 B). 
Whilst in the case where a disc is employed having a central slit only, so 
that no spectral images whatever are admitted (Fig. 5 A), no image at all of 
the lines of the scale is formed (Fig. 5 B). 
From these experiments it may be inferred :— 
1st—The direction of the lines forming the image will be at right- 
angles to the spread of the diffraction spectra ; 
2nd—The formation of an image at all depends upon the admission 
of at least one diffraction spectrum in addition to the 
central beam into the microscope tube ; 
3rd—The formation of a correct image depends upon the admission 
of the whole of the diffraction spectra. 
