demonstrations and explanations relating to the science of optics and light, addressed to the uninitiated, end in plunging them into greater darkness than ever. The only object which the mirror upon the floor serves, in the operation, is to save the workman from the fatigue of holding up the barrel, which it would be necessary for him to do at each observation, if he were to look at the window pane directly. By having a reflecting surface at the floor he can point the barrel downward, when he wishes to look through it, and this greatly facilitates the manipulation. There is a rest, too, provided for the barrel, to support it while the operator is looking through. He plants the end of the tube in this rest, with a peculiar grace and dexterity, and then, turning it round and round, in order to bring every part of the inner surface to the test of the reflection, he accomplishes the object of his scrutiny in a moment, and then recovering the barrel, he lays it across a sort of anvil which stands by his side, and strikes a gentle blow upon it wherever a correction was found to be required. Thus the operation, though it often seems a very difficult one for the visitor to understand, proves a very easy one for the workman to perform. OLD MODE OF STRAIGHTENING. In former times a mode altogether different from this was adopted to test the interior rectitude of the barrel. A very slender line, formed of a hair or some similar substance, was passed through the barrel--_dropped_ through, in fact, by means of a small weight attached to the end of it. This line was then drawn tight, and the workman looking through, turned the barrel round so as to bring the line into coincidence successively with every portion of the inner surface. If now there existed any concavity in any part of this surface, the line would show it by the distance which would there appear between the line itself and its reflection in the metal. The present method, however, which has now been in use about thirty years, is found to be far superior to the old one; so much so in fact that all the muskets manufactured before that period have since been condemned as unfit for use, on account mainly of the crookedness of the barrels. When we consider, however, that the calculation is that in ordinary engagements less than one out of every hundred of the balls that are discharged take effect; that is, that ninety-nine out of every hundred go wide of the mark for which they are intended, from causes that must be wholly independent of any want of accuracy in the aiming, it would seem to those who know little of such subjects, that to condemn muskets for deviating from perfect straightness by less than a hair, must be quite an unnecessary nicety. The truth is, however, that all concerned in the establishment at Springfield, seem to be animated by a common determination, that whatever may be the use that is ultimately to be made of their work, the instrument itself, as