A SILVERED GLASS TELESCOPE. 53 



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

 tion being demanded. 



I have made all the parts of this apparatus so that they can be easily separated 

 or changed. The flat mirrors are of silvered glass, and are used with the silvered 

 side toward the light, to avoid the double image produced when reflection from both 

 sides of a parallel plate of glass is permitted. The large concave mirror happens 

 to be of speculum metal, but it can be repolished if necessary by means of a four 

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

 tarnish easily accumulates on metal specula if they are not carefuUy kept, and de- 

 creases their photographic power seriously. 



Of the maldng of Reverses. — In addition to the use of the Enlarger for magnify- 

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

 of the image of the moon produced in the telescope 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 more occurs. It 

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

 accomplished either by printing from the original negative a positive, Avhich may be 

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

 proofs from. In either case a coUodion film, 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, however, are placed in the cone of sunlight coming from the Enlarger, 

 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 able to see under these circumstances any perceptible diminution 

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

 to use wet coUodion instead of dry plates, no risk of scratching by contact is incurred, 

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

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

 surface or an unsilvered glass mirror in the heliostat. A diaphragm with 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 difl"usion through the room. In enlarging 

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

 half moon for instance, a yellow paper with a half circular aperture, whose size may 

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



The enlarged pictures obtained by this apparatus are much better than can be 

 obtained by any other method known at present. The eff'ect, for instance, of a 

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

 advantages Avould 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 achromatic, before it reached the sensitive plate. But apart from 

 the fact that a reflector would then have all the disadvantages of an achromatic, 



