Fig. 159.—Bull’s-eye Lens.
“During a recent investigation of the spherical aberration in doublets, it was believed to be impossible to construct a doublet of the form known as ‘Herschel’s doublet’ free from aberration, although these doublets figure in many books on optics. In a condenser made by Baker the aberration is reduced to a minimum, 27 per cent. less than Sir John Herschel’s. This doublet, it appears, differs from Herschel’s both in the ratio of the radii of the meniscus, and also in the ratio of the foci of the two lenses; indeed, the only point of similarity is in the first lens, which is crossed. To test this, project the image of the flat lamp-flame on a piece of white card with a plano-convex lens (the field-lens of the Huyghenian eye-piece), use first the convex side and then the plane side towards the card, the lamp being placed about 6 feet from the lens. Focus the lamp-flame as sharply as possible, and a circular halo of misty light will be seen to surround the lamp-flame; but when the plane side of the lens is made to face the card this halo of misty light will be seen to be greatly reduced, and the brightness of the image of the flame proportionately increased. If the lens, then, were strictly aplanatic there should be no misty halo, all the light being concentrated in the image of the lamp-flame, and the image of maximum brightness. In short, the diameter of the halo or misty light is the measure of the spherical aberration. If the condenser referred to above, having the form of minimum aberration for two planes, be compared in the same manner with an ordinary single bull’s-eye of the same focus, the diameter of the misty halo will be found reduced to a radius of about 1⁄5-inch, but, with this new condenser there is a further reduction, so that the radius of the misty halo measures only 1⁄20-inch. These experiments are instructive, because the brightness, or the mistiness of the microscopical image is an associated phenomenon.”[29]
A sectional view of the optical arrangement of Baker’s aplanatic bull’s-eye doublet is shown, together with lamp, in [Fig. 148].
The Microscope Lamp.—The introduction of paraffin into household use has somewhat modified our views with regard to the most suitable artificial source of illumination. Good paraffin burns with a whiter and purer flame than colza oil, and consequently is less liable to fatigue the eyes. The first cost of the lamp is trifling; for a moderate sum a handy form of lamp can be had, mounted on an adjustable sliding ring stand, and with a porcelain, metal or paper shade, to protect the eyes from scattered rays of light. All opticians supply accepted forms of lamps.
To give the increased effect of whiteness to the light (“white cloud illumination” as it is termed), take a piece of tissue paper, dip it into a hot bath of spermaceti, and, when nearly cold, cut out a circular piece and secure it over the largest opening in the diaphragm plate. This will be found to materially moderate and soften the light.
Fig. 160.—Beck’s Complete Lamp.
Beck’s Complete Lamp is constructed especially for delicate microscopical work. It has a burner giving a flat flame; this can be rotated to enable the edge or the flat of the flame to be used; likewise a metal chimney with two apertures, in which 3 × 1 glass slips slide; either white or coloured glasses may be used. A Herschel aplanatic condenser is carried on a swinging arm, which rotates around the lamp flame as a centre, and can be clamped in any position. The whole lamp has a raising and lowering motion, with a spring clamp to hold it in any position. The lamp is so designed that at its lowest position the flame is only three inches from the table. Here the microscopist is furnished with a lamp which will accomplish all he may require with regard to illumination.
Fig. 161.—Watson’s Microscope Lamp.