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.
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.
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.
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.
2, B), since there is no difference in the illumination and other external conditions, --------
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.
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.
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.
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.
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 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.
If a growing stem receives stronger illumination on one side than another, its apex slowly turns from the vertical in the direction of the light source, continuing its change of position until it is in a direct line with the incident rays.
The stem, by pointing directly to the light source, secures the best illumination possible for all of its leaves, the latter being distributed symmetrically around it.
The yellowing and subsequent casting of leaves, for instance, is a very general symptom of disease in plants, and may be induced by drought, extremes of temperature, insufficient or excessive illumination, excess of water at the roots, the action of parasitic Fungi, insects, worms, &c., or of poisonous gases, and so forth; and extreme caution is necessary in.
It may be due to insufficient illumination (Etiolation), as seen in geraniums kept in too shaded a situation, and is then accompanied by soft tissues, elongation of internodes, leaves usually reduced in size, &c. The laying of wheat is a particular case.
In Germany the parallel movement of " illumination " (H.
The illumination is intermittent, and appears to be under the control of the insect's nervous system.