Another method of making glass globules, was de scribed by Mr. Butterfield in the Phil Trans. for 1678. He used the flame of spirit of wine, well rectified, and burned in a lamp ; but instead of cotton he employed small silver wire, doubled up and down like a skein of thread, which being wet with the spirit of wine, and made to burn in the lamp, gives a very ardent flame. Having pounded some glass, and washed it very clean, take some of it up on the point of a silver needle filed very small, and wetted. It must then be held in the flame till it be quite round, and no longer ; and if the side of the glass next the needle is not melted, it may be put off, and taken up with the wetted needle on the round side, presenting the rough side to the flame till it be every where very round and smooth. When wiped with a piece of soft leather, they are ready for being placed between two plates of metal for use.
The ingenious Mr. Stephen Gray proposed to con struct single microscopes with drops of water, in the following manner : I take a thin piece of brass," says he, " filing it into the form A B, (Plate CCCLXXVII. Fig. 1,) making a small hole at A, which serves for an aperture ; then holding it by the other end B, I pour a few drops of water on the table, taking up a small globule thereof with a pin, which I lay on the hole A; then re moving the pin, the water will remain on the aperture, in form of a hemisphere, or a plano-convex lens. But if I have a mind to make a double convex of water, I thrust the pin, which must be less than the hole, through the hole, till the water be entered therein ; then, by draw ing the pin perpendicularly to the plane of the aper ture, the water remains there in form of an aqueous double convex lens. Then, whatever I have a mind to view I take upon a pin, or a piece of glass, accord ing to the nature of the object ; and taking up this na tural microscope by the end 13, I move the object to and fro, till it be in its focus; by which means I can sec objects little less distinctly than by glass microscopes. especially by candle, which I find much better than by day-light.
" But I observed, that those irregular particles which are inherent in the globules of glass were seen dis tinctly, and prodigiously magnified, as was easy to ima gine, both from their nearness to the eye, and hecause they did not hinder the globules, either by thy or can dle light, from appearing throughout transparent, be ing so minute as not to be discernible, except held close to the eye, as in time of observation, and not then neither, it' too near the light, but at a competent distance, they appeared as above. I knew not well how at that time to account for this strange phenome non, that an object should be placed so far within the focus of a sphericic, as to he within the glass, and yet seen distinctly to the eye so near it ; but since, by mat ter of fact, I found it was so, I made this inference, and concluded, that if I conveyed a small globule of water to my eye, and that there were any opaceous, or less transparent particles than the water therein, I might see them distinctly.
" Having by me a small bottle of water, which I knew to have in it some of those minute insects which Mr. Leewenhoek dikovered by the help of excellent mi croscopes; having seen them with the common glass microscopes, and with the first aqueous, as above men tioned, I poured a few drops of this water on the table, and taking a small portion of it on a pin, I laid it on the end of a small piece of brass-wire. I continued to lay on two or three portions of water, till there was formed somewhat more than a hemispherical of water ; then, keeping the wire erect, I applied it to my eye, and standing at a proper distance from the light, I saw them, and some other irregular particles, most enor mously magnified ; for whereas they are scarcely dis cernible by the glass microscopes, or the first aqueous one, within the globule they appeared not much differ ent both in their form, nor less in magnitude than ordi nary peas. They cannot well be seen by daylight, except the room be darkened, after the manner of the famous dioptrical experiment, but most distinctly by candle-light; they may be very well seen by the full moon-light. The pin sometimes takes up the water round enough to show its objects distinct." This mi croscope is shewn fitted up in Plate CCCLXXVII. Fig. 3.
In order to explain the magnifying effect of the globe of water upon the animalcules, or other objects placed in the inside of it, he supposes " the circle in Fig. 2, to represent a sphere of water; A an object placed in its focus, sending forth a cone of rays, two of which are AB, AB, which, coming into the water at B and B, will be refracted from their direct course, and become BD. At D they will, at their passing into the air, be again refracted into DE, DE, and so run parallel to each other, and to the axis of the sphere AECG. Now, as the angle of reflection is equal to the angle of incidence, let the rays BD, BD, be imagined to come from some point of an object placed within a sphere of water, by being reflected from the interior surface of the sphere at BB ; CBD is the angle of re flection, to which making CBF equal, so will F be the place where an object, sending forth a cone of rays, two of which are FB, FB, which are reflected into the rays BD, BD, and then coming to the other side of the sphere at D and D, they are refracted into DE, DE, as before, and consequently be as fit for distinct vision, whether the object be placed at F within, or in A without, the sphere of its interior surface he consi dered as a concave reflecting speculum." Phil. Trans. 1696, No. 221, p. 280.