426 LECTURE XXXVI. 



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 speculums, 

 the one moveable round an axis parallel to that of the earth, and reflecting 

 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 amusement of 

 children, is called a magic lantern: and this, exhibited on a larger scale, and 

 projecting an image on a semitransparent screen of taifetas, instead of a wall, has 

 of late been the source of much entertainment under the name of the phan- 

 tasmagoria, 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 afterwards varnished. By removing 

 the lantern to difl'erent distances, and altering at the same time more or less 

 the position of the lens, the image may be made to increase or di- 

 minish, and to become more or less distinct at pleasure, so that to a person 

 unaccustomed to the effects of optical instruments, 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 thein to appear fainter when they are remote 

 than when they are near: this imperfection might be easily remedied by the 

 interposition of some seraiopaque substance, which might gradually be 

 caused to admit more light as the figure became larger, or by uncovering a 

 larger 6r a smaller portion of the lamp, or of its lens. Sometimes, by throw- 



