NATURAL PHILOSOPHY. 175 



der be reversed end for end, vision will be possible only when the 

 small ends are next the eyes, and the angle of the aperture will be 

 found to subtend exactly the pencil of rays com ing fro in a picture, 

 which is so placed as to l>e bisected at right angles by the plane, of 

 sight. Hence it follows that, the former arrangement of the cylinder 

 being reverted to, the observer looking along the upper side of the 

 aperture will see a narrow : trip extending along the top of the pic- 

 ture ; then, moving the cylinder on and looking along the lower side 

 of the aperture, he will see a similar strip at the bottom of the picture ; 

 consequently, in the intermediate positions of the aperture, the other 

 parts of the picture will have been projected on the retinae. The 

 width of these strips is determined by that of the small ends of the 

 apertures, which measure *1^0 inch; and the diameter of the large 

 ends is I 1 .") inches, the lenses being distant nine niches from the pic- 

 tures. The picture-drum being caused to revolve with the requisite 

 rapidity, the observer will see the steam engine constantly before him, 

 its position remaining unchanged in respect of space, but its parts 

 will appear to be in motion, and in solid relief, as in the veritable ob- 

 ject. The stationary appearance of the pictures, notwithstanding the 

 fact of their being in rapid motion, is brought about by causing their 

 corresponding parts to be seen, respectively, only in the same part of 

 space, and that for so short a time that while in view they make no 

 sensible progression. As, however, there is an actual progression 

 during the instant of vision, it is needful to take that fact into account, 

 in order that it may be reduced as far as practicable, in regulating 

 the diameter of the eye-cylinder, and of the apertures at their small 

 ends. 



OX ARTIFICIAL ILLUMINATION. 



Prof. Frankland, in a recent lecture before the Royal Institution, 

 London, gave the following as the conditions necessary for a good 

 and satisfactory artificial light. In the first place, the light should 

 contain all colors ; that is, it should be capable of showing every va- 

 riety of tint which will be exposed to it. This is the case with the 

 carbon electric light, and that of candle, oil, and gas flames, since the 

 light from these sources contains all the colors of the spectrum. But 

 there are many colors which the mercurial electric light is incapable 

 of showing, since they are absent from its spectrum. It was also 

 shown that all pure colors, except yellow, were perfectly black, when 

 seen by the light of incandescent sodium vapor. Solar light, although 

 in so many respects superior to artificial light, is defective as regards 

 the showing of colors. There are certain colors which cannot be seen 

 by solar light, for instance, all the color which can be seen by the 

 sodium flame is quite invisible by daylight. If a pigment could be 

 made of such a yellow color as to be of exactly the shade of that pro- 

 duced by the sodium flame it would be absolutely black in solar light. 

 But our pigments are all mixed colors, and no such pigment which 

 thus entirely disappears in the light of the sun is known. But in ad- 

 dition to this tint of the sodium flame, there are hundreds of other 

 tints which are also not present in the solar spectrum, and which are 

 consequently invisible in daylight. 



Although solar light is inferior to artificial light in the complete- 



