same appearance as that of the other parties who have not changed their place. In groups of this kind curious effects might be produced by placing a second binocular slide between the principal slide and the eye, and giving it a motion within the stereoscope. The figures upon it must be delineated photographically upon a plate of glass, through which the figures on the principal slide are seen, and the secondary slide must be so close to the other that the figures on both may be distinctly visible, if distinct vision is required for those which are to move. Another method of making solid figures transparent in a photograph has been referred to in the preceding chapter, and may be employed in producing amusing combinations. The transparency is, in this case, produced by using a large lens, the margin of which receives the rays which issue from bodies, or parts of bodies, situated _behind_ other bodies, or parts of bodies, whose images are given in the photograph. The body thus rendered transparent must be less in superficial extent than the lens, and the body seen through it must be so far behind it that rays emanating from it would fall upon some part of the lens, the luminosity of this body on the photograph being proportional to the part of the surface of the lens upon which the rays fall. This will be readily understood from Figs. 48 and 49, and their description, and the ingenious photographer will have no difficulty in producing very curious effects from this property of large object-glasses.