The object will be beautifully depicted on the rough glass: it must be regulated to the focus of the magnifier, by turning the pinion a.
The object may be viewed either with or without the guide for the eye; a single observer will see an object to the greatest advantage by using this guide, which is to be adjusted as we have described, [page 73]. If two or three wish to examine the object at the same time, the guide for the eye must be laid aside.
Take the large lens out of the groove, and receive the image on the rough glass; in this case the guide for the eye is of no use: if the rough glass be taken away, the image of the object may be represented on a paper screen.[29]
[29] A tin cover is sometimes made to go over the glass chimney of the lamp, Fig. 3, with only a small square aperture in front, sufficient to suffer the rays to pass into the microscope: this, by excluding all extraneous rays, adds in many cases most materially to the effect, particularly by day, and when objects are to be represented on the rough glass or screen only. Edit.
Take out the rough glass, replace the large lenses, and use the guide for the eye; attend to the foregoing directions, and adjust the object to its proper focus. You will then see the object in a blaze of light almost too great for the eye, a circumstance that will be found very useful in the examination of particular objects; the edges of the object in this mode will be somewhat coloured, but as it is only used in this full light for occasional purposes, it has been thought better to leave this small imperfection, than by remedying it, to sacrifice greater advantages; the more so, as this fault is easily corrected, and a new and interesting view of the object is obtained, by turning the instrument out of the direct rays of light, and permitting them to pass through only in an oblique direction, by which the upper surface is in some degree illuminated, and the object is seen partly as opake, partly as transparent. It has been already observed, that the transparent objects might be placed between the slider-holders kl of the stage for opake objects, and then be examined as if opake.
Some transparent objects appear to the greatest advantage when the lens at 9 and 10 is taken away; as, by giving too great a quantity of light, it renders the edges less sharp.
The variety of views which may be taken of every object, by means of the improved lucernal microscope, will be found to be of great use to an accurate observer: it will give him an opportunity of correcting or confirming his discoveries, and investigating those parts in one mode, which are invisible in another.
TO TRANSMIT THE IMAGE OF TRANSPARENT OBJECTS ON A SCREEN, AS BY THE SOLAR MICROSCOPE.
It has been long a microscopical desideratum, to have an instrument by which the image of transparent objects might be shewn on a screen, as by the common solar microscope; and this not only because the sun is so uncertain in this climate, and the use of the solar microscope requires confinement in the finest part of the day, when time seldom hangs heavy on the rational mind, but as it also affords an increase of pleasure, by displaying its wonders to several persons at the same instant, without the least fatigue to the eye.
This purpose is now effectually answered, by affixing the transparent stage, Fig. 4, of the lucernal to a lanthorn containing one of Argand’s lamps. The lamp is placed within the lanthorn, and the end 9, 10 of the transparent stage is screwed into a female screw, which is rivetted in the sliding part of the front of the lanthorn; the magnifying lenses are to be screwed into the hole represented at 12; they are adjusted by turning the milled nut. The quantity of light is to be regulated, by raising and lowering the sliding plate, or the lamp. N. B. This part, with its lanthorn and lamp, may be had separate from the lucernal microscope.[30]