given to the plate; then, that a fresh transparency should be inserted, a fresh focusing adjustment made, and a second exposure given, and so on.  This, I say, is conceivable, but it would be very inconvenient.  The adjusting screws would be out of reach; the head of the operator would be in an awkward position; and though these two difficulties might be overcome in some degree, a serious risk of an occasional shift of the plate during the frequent replacement of the dark slide would remain.  I avoid all this by making my adjustments while the plate continues in position with its front open.  I do so through the help of a reflector temporarily interposed between it and the lens.  I do not use the ordinary focusing-screen at all in making my adjustments, but one that is flush, or nearly so, with the roof of the camera.  When the reflector is interposed, the image is wholly cut off from the sensitised plate, and is thrown upwards against this focusing-screen, g.  When the reflector is withdrawn, the image falls on the plate.  It is upon this focusing-screen in the roof that I see the fiducial lines by which I make all the adjustments.  Nothing can be more convenient than the position of this focusing-screen for working purposes.  I look down on the image as I do upon a book resting on a sloping desk, and all the parts of the apparatus are within an easy arm’s length.

My reflector in my present instrument is, I am a little ashamed to confess, nothing better than a piece of looking-glass fixed to an axle within the camera, near its top left-hand edge.  One end of the axle protrudes, and has a short arm; when I push the arm back, the mirror is raised; when I push it forward it drops down.  I used a swing-glass because the swing action is very true, and as my apparatus was merely a provisional working model made of soft wood, I did not like to use sliding arrangements which might not have acted truly, or I should certainly have employed a slide with a rectangular glass prism, on account of the perfect reflection it affords.  And let me say, that a prism of 2 inches square in the side is quite large enough for adjustment purposes, for it is only the face of the portrait that is wanted to be seen.  I chose my looking-glass carefully, and selected a piece that was plane and parallel.  It has not too high a polish, and therefore does not give troublesome double reflections.  In fact, it answers very respectably, especially when we consider that perfection of definition is thrown away on composites.  I thought of a mirror silvered on the front of the glass, but this would soon tarnish in the gaslight, so I did not try it.  For safety against the admission of light unintentionally, I have a cap to the focusing-screen in the roof, and a slide in the fixed body of the instrument immediately behind the reflector and before the dark slide.  Neither of these would be wanted if the reflector was replaced by a prism, set into one end of a sliding block that had a large horizontal hole at the other end, and a sufficient length of solid wood between the two to block out the passage of light both upwards and downwards whenever the block is passing through the half-way position.