ON OPTICAL INSTRUMENTS. 333 



adjust the instrument in the most convenient manner, the distances of all 

 the lenses ought to he moveahle at pleasure : the want of this precaution is 

 a material defect in the usual construction of the instrument. The specu- 

 lum which first receives the light must be capable of motion in all angular 

 directions, in order to allow us to accommodate its position to the change- 

 able place of the sun ; and the adjustment has sometimes been performed 

 by means of a heliostate, an instrument calculated for turning the speculum 

 by clockwork, into such a position as always to reflect the sun's light in 

 the required direction. An easier method would be to employ two specu- 

 lum^ the one moveable round an axis parallel to that of the earth, and re- 

 flecting the sun's light into the direction of its axis, the other fixed, and 

 changing this direction into any other that might be required. When an 

 opaque object is to be examined, the light may be thrown on it either by a 

 plane mirror placed obliquely, or by a perforated concave mirror ; and if 

 the object is small, the concave mirror appears to be the more eligible. 

 (Plate XXVIII. Fig. 400.) 



By night, a lamp with a large lens before it, may supply the place of 

 the sun's light, and the instrument will become a lucernal microscope, 

 which, when painted glass sliders are employed as objects for the amuse- 

 ment of children, is called a magic lantern : and this, exhibited on a 

 larger scale, and projecting an image on a semitransparent screen of taffetas, 

 instead of a wall, has of late been the source of much entertainment under 

 the name of the phantasmagoria, a term which implies the raising of 

 spectres. In order to favour the deception, the sliders are made perfectly 

 opaque, except where the figures are introduced, the glass being covered, 

 in the light parts, with a more or less transparent tint, according to the 

 effect required. Several pieces of glass may also be occasionally placed 

 behind each other, and may be made capable of such motions as will nearly 

 imitate the natural motions of the objects which they represent. The 

 figures may also be drawn with water colours on thin paper, and after- 

 wards varnished. By removing the lantern to different distances, and 

 altering at the same time more or less the position of the lens, the image 

 may be made to increase or diminish, and to become more or less distinct 

 at pleasure, so that to a person unaccustomed to the effects of optical in- 

 struments, the figures may appear actually to advance and retire. In 

 reality, however, these figures become much brighter as they are rendered 

 smaller, while in nature the imperfect transparency of the air causes them 

 to appear fainter when they are remote than when they are near : this 

 imperfection might be easily remedied by the interposition of some semi- 

 opaque substance, which might gradually be caused to admit more light as 

 the figure became larger, or by uncovering a larger or a smaller portion of 

 the lamp, or of its lens. Sometimes, by throwing a strong light upon an 

 actual opaque object, or on a living person, its image is formed on the 

 curtain, retaining its natural motions : but in this case the object must be 

 considerably distant, otherwise the images of its nearer and remoter parts 

 will never be sufficiently distinct at once, the refraction being either too 

 great for the remoter, or too small for the nearer parts : and there must 

 also be a second lens, placed at a sufficient distance from the first to allow 



