1881.] Notes . ■ 375 
the decline of that which in the world corresponds to vitality in 
the individual organism. 
Microscopic Notes . — The influence of the diffraction spedtra in 
the formation of the microscopic image has a far greater bearing 
on the subjedt of errors of interpretation than might be supposed 
by the mere casual observer. If a diatom, such as Pleurosigma 
angulatum , is viewed with an objective sufficient to resolve it, 
and the eye-piece removed, upon looking down the tube a minute 
spot of light — a reduced image of the lamp-flame — occupies the 
centre of the field, and around it, at the extreme margin, six 
equidistant bright specks : these are the diffradtion spedtra of the 
objedt in question, which is adting like a ruled grating. If a 
central stop be placed in the diaphragm of the objective so as to 
cut off the dioptric beam, the diatom will appear with its mark- 
ings resolved, but on a black ground ; but if the diffradlion 
spedtra be intercepted by means of a marginal stop, the diatom 
will be seen on a light ground, but its markings will no longer be 
visible. By using a slit so arranged radially as to include one of 
the diffradlion spedtra, the dioptric beam-lines will appear at right 
angles to the radius ; and by rotating the stop so as in turn to 
include different spedtra, the diredtion of lines may be changed 
at pleasure. This is one of the results of the investigations of 
Prof. Abbe, of Jena, and the whole question of the “ Fundtion of 
Aperture in the production of Microscopic Images ” will be ex- 
haustively dealt with in a paper in preparation for the Royal 
Microscopical Society. It would seem probable that every trans- 
parent tissue may, to a greater or less extent, adt on the illumin- 
ating pencil as a diffraction grating, and consequently produce 
spectral images : what their influence may be upon the image 
formed in the microscope can only be ascertained by a careful 
study of their action. Experiments with ruled lines have shown 
that, by using some of the spectra and excluding others, appear- 
ances totally false may be obtained at will, even the actual known 
direction and the number and distance of ruling of the lines 
being changed. These facts throw very great doubt upon the 
truth of all high-power observations, and it seems probable that 
a careful examination of the diffraction spectra of various tissues, 
with observations of the manner in which the appearance of 
minute structural details undergoes change, will be necessary, as 
a means towards the elimination of errors of interpretation, 
which the results of the researches of Prof. Abbe can leave no 
doubt exist in the whole of our past high-power observations by 
means of transmitted light. 
The number of the “Journal of the Royal Microscopical 
Society” for April contains two important papers on the 
“Aperture” question: one by Prof. Abbe, of Jena, on “The 
Estimation of Aperture ” (a profitable abstract would be almost 
impossible, but the paper deserves careful study by those who 
