HELIOTYPOG'RAPHY (otherwise photoheliography; from Gr. hellos, the sun.) Mr. De la Rue, in the observatory at Kew, has produced, on sheets of paper, pictures in which the solar spots are represented without the aid of drawing or engraving of any kind. In one form of operation (noticed in the Proceedings of the Royal astronomical society), the sun's spots were viewed through a Newtonian reflector of 18-inch diameter, and 10 ft. focal length, producino. an image that would have made the sun's disk 8 ft. diameter. By a nice adjustment, the image of a portion of the disk was received on a glass plate rendered sensitive by collodion. The first part of the process was then complete=the sun painting a picture of his own spots on a piece of glass. Then came the transfer of this negative to a positive, by the usual photographic means of priuting, but with a varnish of very complex chemical nature on the positive plate. This completed the second stage—photography producing a very faint picture on the positive plate.
Then came chemistry; by dissolving away certain constituents of the varnish, which had been more affected than the rest by the actinic force of the sun's light, the surface of the positive plate became a series of ridges and hollows, relied and intaglie, extremely minute in their differences of level, but still sufficiently marked to convey the notion of a kind of engraving. Next came electrotype, or galvanography. The plate, in the state just described, served as a matrix or foundation on which an electrotype cast could be taken. By Prctsch's process, this cast may be so varied as to be available either for surface-printing or for printing on the copper-plate plan. Other solar phenomena, such as the corona, and the appearance presented during total eclipses, have been made to reproduce themselves in a similar way. See also PHOTOGRAPHIC ENGRAVING and PIIOTOGRAPIPi.