The London, Edinburgh and Dublin philosophical magazine and journal of science . esultingspecks in the microscopic image will become imperfectlydark. Diatoms which show the transition from bright specks todark in perfection are those known as Actinoptychus, of whicha good example is almost sure to be found on a St. Peter the half-inch apochromatic and with the iris diaphragmbelow the condenser nearly closed it gives in image x aring of strong diffracted light. Two opposite puncta in thisring and a punctum in the central macula belong to threebeams which produce one of the very numer
The London, Edinburgh and Dublin philosophical magazine and journal of science . esultingspecks in the microscopic image will become imperfectlydark. Diatoms which show the transition from bright specks todark in perfection are those known as Actinoptychus, of whicha good example is almost sure to be found on a St. Peter the half-inch apochromatic and with the iris diaphragmbelow the condenser nearly closed it gives in image x aring of strong diffracted light. Two opposite puncta in thisring and a punctum in the central macula belong to threebeams which produce one of the very numerous rulings whichconspire to form the specks in the microscopic image. Eachsuch trio of beams furnishes both sets of the dotted lines infig. 2 on p. 515. And as the ring is a tolerably circular onethe other similar trios, whether in the same or in otherlongitudes, furnish dotted lines in fig. 2 that are not veryfar from being equally inclined. These are conditions thatwill produce black specks at the height represented by p inPhil. Mag. S. 5. Vol. 42. No, 251). Dec. 1896. 2 0. Image x. 518 Dr. G J. Stoney on Microscopic Vision. fig. 2 on p. 515. Now open the iris diaphragm, insert theeyepiece, and look at this diatom : it will be found to exhibitthe phenomenon with great distinctness. A modification of this experiment is to cut a piece of cardof the annexed form and to place it over the back of the objective. It allows only ^ ^^ two of each trio of beams to pass. These / ^_^^ \proceed to form the rulings which are I ( ) \competent to form specks in the micro- *—* *—scopic image ; and accordingly that imagewill still exhibit the bright specks, which, however, will nowgo out of focus without any dark specks appearing. Notice that the specks now appeal to traverse in a directionperpendicular to the diameter of the semicircular stop, whilethe microscope is being put out of focus. This is because thedotted lines of fig. 1, p. 514, are here necessarily oblique. 39. Experiments ex
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