The electric lamp a gives illumination of the webs in a dark field, nearly in the manner described for the Cape transit circle micrometer; the intensity of illumination is regulated by a carbon-resistance controlled by the screw b.
Forty times, it is said, he read through the Metaphysics of Aristotle, till the words were imprinted on his memory; but their meaning was hopelessly obscure, until one day they found illumination from the little commentary by Farabi, which he bought at a bookstall for the small sum of three dirhems. So great was his joy at the discovery, thus made by help of a work from which he had expected only mystery, that he hastened to return thanks to God, and bestowed an alms upon the poor.
Owing to the variable illumination of the selenium thus produced, the resistance of the latter, and therefore the intensity of the current sent through the line to the receiving station by the battery, will be altered accordingly.
This is the main transpiring tissue, and is protected from direct illumination and consequent too great evaporation.
The most constantly occurring changes that beset a plant are connected with illumination, temperature, moisture, and contact with foreign bodies.
Cases of pallor due to too intense illumination and destruction of chlorophyll must also be distinguished.
The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a lens, or with lens-shaped thickenings of the cuticle, through which convergence of the light rays takes place and causes a differential illumination of the lining layer of protoplasm on the basal walls of the epidermal cells, by which the stimulus resulting in the orientation of the leaf is brought about.
The process of augmenting the resultant illumination at a particular point by stopping some of the secondary rays may be carried much further (Soret, Pogg.
Such a plate has the power of a condensing lens, and gives an illumination out of all proportion to what could be obtained without it.
From the general formula (2), if A be the area of aperture, 102 = A2 / x2 f (7) The formation of a sharp image of the radiant point requires that the illumination become insignificant when, n attain small values, and this insignificance can only arise as a consequence of discrepancies of phase among the secondary waves from various parts of the aperture.
When the difference of phase amounts to A, we may expect the resultant illumination to be very much reduced.
In like manner we may find the illumination in any other direction, and it is obvious that it vanishes when sin 0 is any multiple of A/a.
Thus, if x = R cos 4), C =,r2R2J1(pR) pR and the illumination at distance r from the focal point is 4T2 r 21rRr1 fX (2 fKr) a J The ascending series for J 1 (z), used by Sir G.
We will now investigate the total illumination distributed over the area of the circle of radius r.
If r, or z, be infinite, Jo(z), J 1 (z) vanish, and the whole illumination is expressed by 71-R 2, in accordance with the general principle.
In any case the proportion of the whole illumination to be found outside the circle of radius r is given by J02(z)+J12(z).
For the dark rings Ji(z) =o; so that the fraction of illumination outside any dark ring is simply Jo 2 (z).
In estimating theoretically the resolving power on a double star we have to consider the illumination of the field due to the superposition of the two independent images.
The illumination at B due to P then becomes comparatively small, indeed for some forms of aperture evanescent.
The only effect of the ruling is to diminish the amplitude in the ratio a: a+d; and, except for the difference in illumination, the appearance of a line of light is the same as if the aperture were perfectly free.
On either side of any one of them the illumination is distributed according to the same law as for the central image (m = o), vanishing, for example, when the retardation amounts to (mn t 1)X.
The expression (5) gives the illumination at due to that part of the complete image whose geometrical focus is at =o, the retardation for this component being R.
As the point P is more and more deeply immersed in the shadow, the illumination continuously decreases, and that without limit.
We have ultimately G =o, H = (7rV)- 1, so that 1 2 = I / 12V 2, or the illumination is inversely as the square of the distance from the shadow of the edge.
We will next suppose that the light is transmitted by a slit, and inquire what is the effect of varying the width of the slit upon the illumination at the projection of its centre.
If the slit is of 'constant width and we require the illumination at various points on the screen behind it, we must regard the arc of the curve as of constant length.
If the eye, provided if necessary with a perforated plate in order to reduce the aperture, be situated inside the shadow at a place where the illumination is still sensible, and be focused upon the diffracting edge, the light which it receives will appear to come from the neighbourhood of the edge, and will present the effect of a silver lining.
Thus, rays suffering one internal reflection will all lie within a cone of about 42°; in this direction the illumination will be most intense; within the cone the illumination will be fainter, while, without it, no light will be transmitted to the eye.
The inhabitants of tropical America sometimes keep fireflies in small cages for purposes of illumination, or make use of the insects for personal adornment.
The illumination of the field is given by a lamp near the object glass, controlled by a switch near the micrometer.
Their absolute freedom from diffraction, the perfect control of the illumination and thickness of the lines, and the accuracy with which it will be possible to construct scales for zone observations will be important features of the new method.