OPTICAL PROJECTION 182 dependent approval, especially in first-class public institutions ; and, so far as I can learn anything at all about them—which it has been most difficult to do—their success has been in precise proportion to the degree in which the same general arrangements have been adopted, though I have not as yet heard of equal results having been obtained. I therefore con¬ fine myself to what I know and have myself openly tested in various public demonstrations. Moreover, everything in this chapter except the description and explanation of the instru¬ ment in detail, will apply equally to others in proportion to the efficiency of their performance. 92. The Oxy-hydrogen Microscope.—Fig. 102 gives a sec¬ tion of the instrument as constructed for the oxy-hydrogen light. u represents the lantern condenser, which 1 prefer to make y inches in diameter and of triple form, so as to take up an angle of 95°, and bring the rays to a focus, if let alone, at about G inches in front of the front lens. If the microscope be required to fit an ordinary 4-inch lantern front, the arrange¬ ment must be slightly different, the lime being in this case pushed up so as to give an approximately parallel beam with the ordinary double condenser, while a third lens either slides or racks in the back-end of the microscope, so as to bring the parallel rays to a focus corresponding in character to the