In 1849, Bourdon invented the steam pressure gauge that still bears his name, and made a contribution of distinct and permanent value, by which ability to keep track of the steam pressure in boilers was increased, and safety from explosion increased proportionately. In the same year, Sir David Brewster invented his lenticular stereoscope. In this beautiful instrument two separate pictures of the same object are put on one card, one picture showing the object as it would look to the left eye from a given distance, and the other picture showing the object as it would look to the right eye. The two eyes of an observer look at the two pictures through the two halves of two convex lenses, that are so shaped that the two pictures are seen as one picture, but so superposed as to represent the object in relief, as the actual object appears to the two eyes. Like the kaleidoscope, this later product of Sir David Brewster's brilliant imagination has had little influence thus far, except possibly to lead the way toward stereo-photography and the stereopticon: but it seems hardly probable that an important field will not be found some day for an invention so suggestive. In the same year, Hibbert made an important improvement on the knitting machine, and Corliss invented his famous engine cut-off, which vastly economized fuel. Neither invention was especially novel or brilliant, but both were highly practical and useful contributions to the improvement of the Machine. In the same year also came Worm's improvement on the printing press, that concerned the making of "turtles" which held type in a curved shape, so that they could be secured to the cylinder of the press. In 1850, Scott Archer succeeded in using collodion to fix silver salts on the surface of glass plates in photography. He cannot be credited with the basic invention, because the idea of doing this had been suggested long before. The invention made an important contribution to the growing art of photography, mainly by supplying a stepping stone for further advances. In the same year, an important improvement was made in watch-making by inventing a watch-making machine. This was one of the first of those distinctly American inventions, by which machine-work replaced hand-work, with great increase in speed of production and lessening of cost, but without decrease in accuracy of workmanship.