" Proper illumination is the most difficult thing to obtain, and withal the most indipensable. An unmixed ray from the edge of the flame, passing perpendicularly through the filter-cell and evenly converged on the object by a good bull's eye with the flat side turned toward the stage of the microscope, will give a large, level field of well-focused definition. A broken ray, full of cross refractions and unevenly condensed, shows only a small patch of moderate sharpness, and even in that, spectral diffraction borders are apt to appear. These ghosts will sometimes spoil an otherwise perfect negative when the flame wavers in a draught. A critically' disposed light is surprisingly different in its effects from one uncritically' adjusted. The minute hairs on a house-fly's wing afford a simple test— under a quarter-inch objective, if badly lighted, they show double points diverg ing from their base.
" The fine adjustment of the micro scope must be attached to some ade quate contrivance so as to be readi ly operated while the observer stands before the ground-glass screen. A side rod, with milled head at the hither end and a pulley wheel at the other, from which a thread band passes over the milled head of the micrometers crew, is pretty, but bothersome to apply, and not as sensitive to manipulate when the higher powers are employed as is the more primitive device of a stout thread with a weight at each extremity. This is waxed in the middle where it runs over the micrometer head, and thence passes through guide rings at the sides of the general base-board to the opera tor's hands, where the weights hang right and left, convenient to the slight est touch.
" This inexpensive apparatus permits of really excellent photographs of thin sections being taken with objectives of from two inches to one-third inch focus. The resultant magnification depends, obviously; on the distance of the screen from the objective. I find thirty inches a favorable length, as it gives a circle of nearly four inches. If a greater distance be essayed, the light will be corres pondingly diminished and longer exposures required, while the gain in amplification is balanced by some loss of sharpness.
"For the higher powers, one quarter to one-eighth inch, a good substage condenser should be used on the microscope itself, and the bull's eye discarded. The common non-achromatic Abbe condenser works fairly well, especially if the angle of the illuminant cone be dimin ished by removing the smaller top-lens of the combination; but for critical ' work an achromatic Abbe must be used. With its
employment, however, as with marriage according to the old rhyme, one's trouble begins, for it will not do its best unless the pencil of light impinging on its lower face be practically parallel. How to get a straight ray of strong lamp-light is a pro blem which the makers have not yet volun teered to solve satisfactorily, but it should not be a hard one. Sun-light and a calcium burner, both of which throw parallel rays, being out of the question for me, I have taken a powerful projection lantern and set it as near to the microscope as the interven ing heat-filter will permit. As the beam from its projection-lens is a divergent cone, I necessarily lose the greater part of the emitted light, but that which follows the axial centre (no bull's eye being used) proves sufficient for my needs, and the achromatic Abbe condenser does the rest. What I want is a substitute lantern-objective which will collect all the light from the four and-a-half-inch condenser and project it in a parallel ray about an inch in diameter upon the base of the Abbe.
"Equally with correct illumination, a good objective is essential to creditable work. One of the very best I have is a Bausch & Lomb half-inch with an iris diaphragm set in its tube between the two combinations. Another excellent glass is a Reichert 8-millimeter ' half apochromat,' which shows a larger and flatter field than the full apochromats, giving at 3i inches sharp four-inch circles with a magnification of too diameters. There is no occasion for the mere amateur to spend a small fortune on the high-priced apochromats, since almost any first-class achromat of reputable Ameri can or English make will do the business as well, even though not especially corrected for photography. Besides being, from the peculiarities of construction, liable to mishap or deterioration, the apochromat field is pur posely smaller and less flat, needing compensa tion by a specially computed eye-piece; and they work slower. Having an excessive numeri cal aperture, their working distance is narrow and with a conjugate focus of 3o inches to the screen the frontal focus is inversely lessened, so that only the thinnest covers can be made use of with the highest powers. They do their best, and very good it is too, with the aid of expressly constructed projection oculars, which flatten the field besides length ening the working-distance—but this entails such diminution of light as to make accurate focusing difficult by the comparatively feeble rays of a lamp.