248 FERGUSON'S LECTURES. 



LECT. Before we enter upon the description of telescopes, 

 \^-v-^_, it will be proper to shew how the rays of light are af- 



Telescopes. 



it has a longer focus. A small drop or spherule of water, held to the eye 

 by candle light or moonlight, without any other apparatus, magnifies in a 

 very surprising manner the animalculae contained in it. The reason is, 

 that the rays, coming from the interior surface of the first hemisphere, 

 are reflected so as to fall under the same angle on the surface of the pos- 

 terior hemisphere, to which the eye is applied, as if they came from the 

 focus of the spherule ; whence they are propagated to the eye in the same 

 manner as if the objects were placed without the spherule in its focus. 



These water microscopes have given rise to the use of other fluids, with 

 several varieties of construction. Dr.Brewster has described one in whicli he 

 makes use of very pure and viscid turpentine. This he takes up by the point 

 of a piece of wood, and drops successively upon a thin and well-polished 

 glass : different quantities being thus taken up and dropped in a similar 

 manner, form four or more plano-convex lenses of turpentine varnish, 

 which may be made of any focal length, by taking up a greater or less 

 quantity of the fluid. The lower surface of the glass having been first 

 smoked with a candle, the black pigment below the lenses is then to be 

 removed, so that no light may pass by their circumference. The piece of 

 glass is then to be perforated, and surrounded with a toothed wheel, whicli 

 can be moved round the hole as a center by an endless screw. The ap- 

 paratus is then placed in a circular case, and this case fixed to a hori- 

 zontal arm by ineams of a brass pin, which passes through its upper and 

 under surfaces, and through the hole already mentioned, which does not 

 embrace the pin very tightly, in order that the toothed wheel may re- 

 volve with facility. On the upper surface of the circular case is an 

 aperture directly above the line described by the centers of the fluid 

 lenses, when moving round the central hole ; and in this aperture is in- 

 serted a small cap, with a little hole at its top, to which the eye is applied. 

 A inoveable stage carries the slider, on which microscopic objects are 

 laid, and is brought nearer or removed from the lenses by a vertical scre\v. 

 The objects on the slider are illuminated by a plane mirror, which has 

 both a vertical and horizontal motion for this purpose. When the mi- 

 croscope is thus constructed, the object to be viewed is placed upon the 

 slider, and the endless screw is turned till one of the lenses be 

 directly under the aperture ; and the slider is then raised or depressed 

 by the vertical screw, till the object be brought into the focus of the lens. 

 In this manner, by turning the endless screw, and bringing all the lenses. 

 one after another, directly below the aperture, the object may be succes- 

 sively examined with a variety of magnifying powers. These fluid 

 lenses have been employed as the object-glasses of compound mi- 

 croscopes. 



Minute glass spherules make very excellent microscopes, to those who 

 have a little patience in using such instruments; for the foci of the 



