Translation of Professor Abbe's Paper on the Microscojpe. 245 
sexual, the germs of which, there as here quiescent in the mud, 
wait to be revivified by the increasing warmth of next year's sun. 
Unhappily I never succeeded in again discovering the Trocho- 
sphaera either at Manila or on Bohol, so that I must leave it to 
future visitors of Zamboanga to solve the questions here proposed, 
and to discover really sexual specimens of this the most interesting 
of all Eotatoria. — Siebold and Kblliker's Zeitsclirift, drittes Heft. 
Y. — Extracts from Mr. H. E. Fripp's Translation of Professor 
Abbe's Paper on the Microscope. 
(Continued from p. 201.) 
The undulation theory of light demonstrates in the phenomena 
of diffraction a characteristic change which material particles, 
according to their minuteness, effect in transmitted rays of light. 
This change consists, generally, in the breaking up of an incident 
ray into a group of rays with increased angular dispersion within 
the range of which periodic maxima and minima of intensity (i. e. 
alternation of dark and light) occur. But these angular distances 
are for each colour proportional to its wave-length, and increase, 
therefore, in size from violet to red, and are also inversely propor- 
tioned to the distances between the particles in the object which 
cause diffraction. 
This effect, which is not only such as might be theoretically 
predicted, but also capable of exact calculation, may be readily 
observed. Having placed some object of the kind in question 
under the microscope and got its detail in focus, the ocular must be 
removed and the image of the object in the open tube viewed with 
the naked eye, or a suitably arranged microscope of weak power 
(V to 1°) which can be let down in the tube to the upper focal 
plane of the objective. The image of the mirror or whatever illu- 
minating surface may be used will be seen as it is formed by the 
undiffracted rays, and surrounded by a greater or less number of 
secondary images in the form of impure coloured spectra, whose 
sequence of colours, reckoning from the primary image, is always 
from blue to red. Objects consisting of several systems of lines 
which cross each other show not only a series of diffraction images 
of each group in the direction of their perpendicular, but also other 
additional series in the angles between the perpendicular groups. 
Insect scales and diatom valves exhibit these phenomena in the 
greatest variety. 
This method of direct observation of pencils of light coming 
from the object enables us to determine by experiment what part 
