§6 local particttlaes of the transit of ventjs, 1874. 



Mb. Eittheefoed to Ameeican Commissionees. 



"I think I can best reply to yovir letter of tlie 19tli instant, making 

 inquiries as to the application of photography to the transit of Venus, by 

 describing the methods now and for the last three years in use at my observa- 

 tory for photographing the sun, with results which I have not seen equalled 

 by any other process. 



" 1st. The object glass is corrected for photographic rays without reference 

 to the effect on the visual image. This correction I have effected in two 

 modes ; first, by constructing a double objective of fl.int and crown glass with 

 such curves as will produce the. correction ; second, by applying to an ordinary 

 achromatic objective a lens of such curves and density as will produce the 

 requii'ed correction. Of the first description is the lli-inch, now the property 

 of the Argentine Repubhc ; of the second description is the 13-inch objective 

 now mounted in my observatory. 



" Without a proper correction the photographic images are without sharp- 

 ness, and consequently entirely unfi.t for measurement. {See note at end.) 



"With an objective of the above description it is not possible to adjust the 

 focus by the eye. This must be done by trial, and there must be a micrometer 

 screw, so that the true focus once found may be recovered at pleasure. I use 

 a galvanized iron tube with three thermometers, from whose reading the focal 

 adjustment is made by the screw for the change in tlie length of the tube. 

 I find that stars fiimish the best means of determining the focus. 



" The camera box is arranged so that a plate-holder may be inserted, and 

 a picture taken at the focus of the objective. The size of a picture so taken 

 is about one-tenth of an inch diameter for each foot of focal length, and is too 

 large to permit an instantaneous and miiform exposure. The image of the 

 sun is enlarged by a photographic camera lens of ordinary construction. I use 

 my 13-inch Harrison i-plate portrait tube. No doubt a better enlarging lens 

 could be devised with a flat field and corrected for photographic rays alone ; 

 whereas the best portrait lenses have considerable curvature of field, and 

 profess only to unite the photographic and visual rays ; a compromise which 

 sacrifices about one-half of the sharpness of each. After traversing this com- 

 bination the rays cross on their way to the sensitive plate. The j)oint where 

 the rays cross is so small that a very simple and hght snap for instantaneous 

 exposure is possible, and an additional and very important advantage is 

 seciu'cd in that the whole image is simultaneously exposed and cut off. 



" Directly in front of the sensitive plate is a very fine platina wire, stretched 

 east and west with a simple adjustment, by which it maj be niade to coincide 

 vrith the course of a star or sun spot ; the shadow of this wire will mark on 

 the photo the zero of position. Of course this supposes the instrument 

 accurately adjusted to the meridian and the elevation of the pole. Behind 

 the plate, with a motion from side to side, is mounted a Ramsden eye-piece, 

 for the purpose of finding the angular value of any part of the plate. 



" The adjustments required are : — 1. Collimation of objective. The object- 

 glass must be adjusted by the proper screws until the flame of a candle held 

 at a small hole in the middle of the plate-holder, is seen reflected, and super- 

 imposed by an eye looking through the blue transparent part of the caudle. 

 2. Collimation of the plate. This is effected by putting in the plate-holder 

 a piece of plate-glass smoked on the side most distant from the object-glass 

 and covered on the other side all but the central quarter of an inch. A cap 

 with a quarter-inch central hole is then put over the objective, and when the 

 mage of a candle held before this opening is reflected back through it by the 

 smoked-glass, the plate-holder is in collimation. The enlarging lens must 

 also be put in collimation by proper screws, using the reflection of a candle in 

 the same way as for the object glass. 



