A S I L V E R E D G L A S S 1' E L E S C O P E. 53 



accuracy that the optical arrangement allows, no correction for chromatic aberra- 

 tion being- demanded. 



I have made all the ])arts of this ajjparatns so that tliey can be easily separated 

 or eJianged. 'I'lie Hat mirrors are of silvered glass, and are used with the silvered 

 side toward the light, toa\oirl the d()nl)lc image produced when reflection from both 

 sides ot a iiarallel plate of ghiss is permitted. The large concave mirror happens 

 to be of specnlum metal, bnt it can be repolished if necessary by means of a four 

 inch polisher, passed in succession over every chord of the face. A yellow tilm of 

 tarnisli easily accumulates on metal specula if they are not carefully kept, and de- 

 creases their photographic power seriously. 



Of the vHikliHj of Rererses.- In addition to the us(> of the Enlnrger for nnignifv- 

 ing, it is found to have important advantages in copying by contact. The picture 

 of tlie image of the moon produced in tlie t(descope is negative, that is, the lights 

 and shades are reversed. In enlarging such a negative reversal again takes place, 

 and a positive results. This positive cannot, however, be used to make prints on 

 paper, because in that operation reversing of light and shade once nnjre occurs. It 

 is necessary then at some stage to introduce still another reversal. This may be 

 accomplished eithiT l)y printing from the original negative a positive, which may be 

 enlarged, or else printing from the enlarged positive a negative to make tire paper 

 proofs from. In either case a c:ollodion tilm, properly sensitized, is placed behind 

 the positive or negative, and the two exposed to light. 



If diffused light or lamplight is used, the two plates must be as closely in contact 

 as possible, or the sharpness of the resulting proof is greatly less than the original. 

 This is because the light finds its way through in many various directions. If the 

 two plates, howe\er, are placed in the cone of suidight coming from the Eidarger, 

 and at a distance of fifteen or twenty feet from it, the light passes in straight lines 

 and only in one direction through the front picture to the sensitive plate behind. 

 I have not been alile to see under these circumstances any perceptible diminution 

 in sharpness, though the plates had been -j',. of an inch apart. It is perfectly feasible 

 to ixse wet collodion instead of dry plates, no risk of scratching by contact is inciuTed, 

 and the whole operation is easily and quickly performed. The time of exposure, 5 

 seconds, is of convenient length, but may be increased by jjutting a less reflecting 

 surface or an unsilvered glass mirror in the heliostat. A dia])hragm Avith an aper- 

 ture of half an inch if placed at e, Fig. 46, to shut out needless light, and avoid 

 injuring the sharpness of the reverse by diffusion through tlie room. In enlarging 

 other diaphragms are also for the same reason put in the plac-e of this one. For a 

 half moon for instance, a y-ellow paper with a half circular apcn'ture, whose size may 

 be found by trial in a few minutes, is pinned against c 



The enlarged pictures obtained by this apparatus ar(> much better than can be 

 obtained by any other method known at present. The effect, for instance, of a 

 portrait, made life-size, is very striking. Some astronomers have supposed that 

 advantages would arise from taking original lunar negatives of larger size in the 

 telescope, that is, from enlarging the image two or three times by a suitable eye- 

 piece or concave nchroiuiilic, liefore it reached the sensitive plate. But apart from 

 tlu' fact tluit a retl(>ctor would then have all the disadvantages of an achronuitic, 



