SPECTRUM ia made; and so on, the machine working automatically until the plate is ruled. The pitch of the screw is ^ of an inch ; hence, by knowing the fraction of the revolution of the screw made between two contiguous cuts, we know the distance, in fraction of -fa of an inch, separating the centres of two contiguous lines on the grating. The diameter of the feed wheel B is 6 in., and from this dimension the reader may estimate the size of the other parts of the engine. The excellence of the work done by this engine depends on certain peculiarities of construction : 1. The errors in the throw of the screw, caused by its eccen- tricity, or want of coincidence of the axis of figure of the screw and its axis of motion, are corrected by giving to the feed wheel B an eccentricity opposed to that existing in the screw. The screws at C, C, C, serve to alter the position of the centre of the feed wheel B, and thus to obtain the required ec- centricity. This balance of opposing eccen- tricities is obtained by a delicate and precise method devised by Mr. Rutherfurd. A gra- ting ruled by the engine on speculum metal or silvered glass is examined at such an angle of reflection that the light reflected from the plate appears of one color ; for example, red. If we could cause the lines on this grating gradually to approach each other and narrow the spaces separating them, we should observe the red tint becoming gradually lighter in hue, and then gradually changing into orange, yel- low, green, &c. Hence uniformity of tint is an indication of uniformity of ruling ; there- fore, if we tilt a grating placed in front of a flame and examine it by means of a lens and find it of a uniform color, we are sure that the screw is doing accurate work ; but if the screw have a periodic error like ec- centricity, then we shall observe a columnar appearance on the plate, owing to periodic variations in depth of color. By a careful comparison of the positions of these colors with the known positions of the screw when the corresponding portions of the grating were cut, we ascertain the direction in which to move the centre of the feed wheel B in order to correct the eccentricity of the screw. 2. The nut in which the screw of this engine works is 3 in. long, the threaded portion of the screw being 3$ in. long. This long nut tends to preserve the accuracy of the engine's work. 3. The original method devised by Mr. Rutherfurd to obtain rectilinear V guides is a new and important feature of this engine. A collimating telescope with a vertical slit, or cross threads, at the focus of its object glass, is placed in a firm position in a line with the guide to be tested. On the slide which moves on the V guide is fastened a telescope with cross threads at the focus. The slit of the collimating telescope is viewed in the otlu-r telescope as the slide carrying the lat- ter is moved to and from the collimator. If the slit ia constantly bisected during this mo- tion, the guide is accurate ; but if the image of the slit moves when referred to the cross threads, then the guide has to be corrected until by trial the image of the slit remains stationary during the telescope's motion. 4. The pawl H during its retraction does not fall over the teeth of the feed wheel, and by jar- ring the machine cause the diamond to cut an irregular or waving line. 5. The method of lifting the diamond point while it is obtaining its position for a new cut is the simplest we know of. 6. The screw of this engine, on which the results mainly depend, is construct- ed as follows : A screw is first cut in a lathe with a single pointed tool, then scored ; it is then hardened, and thus a tap is obtained. This tap is now centred by its threads, and cylinders are ground on its ends so that they are concentric with the threads of the screw. Blocks of metal are now firmly screwed on to the bed plate of a planer, and Vs are cut in these blocks to support the cylindrical ends of the screw tap. A stock holding blank dies is placed in the tool holder of the planer, and these dies are screwed against the screw tap. The stock is then firmly screwed to the tool holder. The threads of the dies are now cut by rotating the screw tap two or three times through the dies ; then, relieving the die stock in the tool holder, the dies are again tight- ened on the screw tap; the stock is now screwed tightly on the tool holder, and the screw tap is again traversed through the dies. This operation is repeated until the dies are finished, when they are hardened. A screw which is to serve for the ruling engine is now cut on the lathe with the same single-pointed tool which cut the thread of the master tap. This screw is nearly finished on the lathe. It is then placed in the same Vs which previous- ly held the master tap, and the dies, just de- scribed, are placed in the tool holder of the planer, and with them the thread of the screw is finished. The screw is now rotated on its threads in a long cast-iron V, and shoulders are turned down on the ends of the shaft of the screw. The nut for this screw is cut with a single-pointed tool. The screw is now run into its nut, and they are ground together with finely powdered pumice stone. The screw of this engine has only one collar, on which it turns near the feed wheel B. The other sup- port of the screw is the long nut. Without this arrangement Mr. Rutherfnrd found it im- possible to cut regular gratings. Actions of the Spectral Rays. A large mass of evidence shows that all of the known emanations from the sun consist of rapid vibrations caused by that luminary in a highly elastic medium, known as ether. We must suppose that this ether fills all known space, for we can only be cog- nizant of celestial bodies from their vibratory actions on the ether through which they con- stantly move. The nature of the manifesta- tions of these ethereal vibrations will depend on the nature of the bodies on which they
