1VK SCIENCE. 



.., t!n-y will be seen painted on the paper, In.' 

 ilit to left. 



It may be sometime* useful to make u drawing when the 

 objects are transposed in thin manner : for example, in the oane 

 where it is inttnuled to be engraved, for as the imprtH*i< i 

 plate will transpose the figures from right to left, they will then 

 appear in their natural potation. 



('t.i / succession 



all th<- objects in the neighbourhood 

 mZ quite round the machine. 

 Place the mirror H in a vertical 

 position, as seen in Fig. 1, and 

 incline the mirror i. at an angle of 

 45 degrees ; if the former bo then 

 turned round vertically, the lateral 

 ' ! will be soon painted in 

 succession on the paper, in a very 

 pleasant manner. 



It must hero be observed, that 

 it will be necessary to cover the 

 mirror H with a kind of box mode 

 of pasteboard, open towards the 

 objects, and also towards the aper- 

 ture N of the box x ; for if the 

 mirror H wore left entirely exposed 

 it would reflect on tho mirror L a great many lateral rays, 

 which would considerably weaken the effect. 



In tho arrangement of tho photographic camera every care 

 is taken to exclude lateral rays, so as to allow only those to 

 enter tho camera which come direct from the object intended 

 to be delineated ; so also in obtaining a sharply-defined picture, 

 the whole aperture of the lens 

 is not left open, but carefully 

 closed by stops of various sizes. 

 All these niceties, which might 

 be thought to belong only to 

 the modern practice of photo- 

 graphy, were perfectly under- 

 stood and made use of by Grave- 

 sande, who describes them in 

 the following paragraph : 



(4.) To represent ' tlic image 

 of paintings or prints. Affix 

 tin 1 painting or print to the side 

 of the board F which is opposite 

 to the mirror L, in such a manner 

 that it may bo illuminated by 

 the sun. But as the object in 

 this case must be at a very 

 email distance, the tube should 

 be furnished with a glass, having 

 its focal distance nearly equal 

 to half the height of the machine 

 above the paper ; if the distance 

 of the painting from the glass 

 be then equal to that of tho 

 gloss from the paper, the figure 

 of the painting will bo repre- 

 sented on the paper exactly of 

 the same size. Some attention 

 is necessary in regard to the 

 aperture of the convex gloss. 

 In the first place, it may have 

 the same opening as that of a 

 telescope of the same length. 

 Secondly, the opening must be 

 diminished when the objects are 

 very much illuminated, and vice 

 versa. It will, therefore, be 



proper to have different circles of copper or of blackened 

 pasteboard, to be employed for altering the size of the aperture 

 according to circumstances. 



Hod Qravesonde been born in more modern times, he would 

 no doubt have made his name celebrated by constructing a 

 camera for photographic purposes quite as ingenious and 

 complete as the one he devised only for those who used no 

 chemicals, but worked laboriously with pencil and paint-brush. 



The camera lucida is another optical contrivance by which 



the Image of any object U projected on to a nheet of paper to at 

 to enable an artist to make an accurate traeifg f 

 instrument was invented, in the year 1607, by the Can" 

 Wolhwton, and owes its ruefulness to the internal reflection of 

 light ; it comujtU of a four-sided prism mounted OB a bra** 

 BHM. 



In Fig. 2 the quadrangular- uapod prism is shown at Am DC; 

 the angle most be W, D 671, 

 and o 136 : if an object o o be 

 placed opposite the face B u the 

 rays of light proceeding from O O 

 will, after falling on DC, be re- 

 fleeted inside the glass to c A, and 

 thence to the eye at . There is 

 no image actually formed on the 

 sheet of paper placed below the 

 prism at P P, like that on the 

 ground-glass screen of the camera 

 obscura ; bat, as all objects appear 

 to come in a straight line from 

 the point whence they are seen, 

 the imago will appear as far below 

 the instrument as the object is 

 before it, and it will, therefore, 

 seem to be delineated on the paper 

 p P. The prism is covered at the top with a metallic eye-piece, 

 the hole of which lies half over tho edge of the prism, ""* by 

 this simple arrangement both the image and the paper become 

 visible to the draughtsman, who would not otherwise be able to 

 see his pencil and make tho outline of the object depicted on PP. 

 Amici, an Italian philosopher, invented two other and more 

 ingenious modifications of the 

 original camera lucida. The 

 first (Fig. 3) consist* of a paral- 

 lel piece of plate-glass, A B, 

 joined to a metallic reflecting 

 speculum, c, which is inclined to 

 the surface of the glass A B at 

 an angle of 135. The rays 

 from the object at D are reflected 

 by the speculum to the glass at 

 E, and this "ghost-like" re- 

 flection passes to the eye. 



In Fig. 4 the rays pass 

 through the plate-glass before 

 they fall upon the metallic reflec- 

 tor. The ray from the object A 

 passes in a straight line through 

 the glass, and being reflected at o 

 from the speculum B, it is again 

 reflected in true "ghost fashion" 

 from the surface of the plate- 

 glass at c to tho eye at s. 



Amici' s arrangement is most 

 convenient to the artist, who 

 will always be able to see the 

 image, the paper, and his pencil, 

 whereas with Wollaston's origi- 

 nal instrument considerable pa- 

 tience and practice is required 

 before the draughtsman can use 

 it properly. 



A very comprehensive work 

 on "transparent mirrors" has 

 been written by M. Lucas, an 

 engineer of permanent ways and 

 bridges in France, and he de- 

 tails some modifications of the 

 " ghost illusion " combined with 

 the effects of persistence of vision, by which singular optical 

 illusions are produced. A clever abbreviated translation iu 

 the Building Nevus describes what takes place. 



Divide a circle into two n sectors, of which n has a central 

 angle a, and n has a central angle b, the sectors a and 6 

 succeeding alternately. Cut out the sectors 6 so as to form 

 spaces. A sort of star will remain, formed by the n sectors a, 

 on which either silvered glass or, what is preferable, metallic 

 plates, silvered over and polished, are to be placed. This 



