NELSON'S LAMP 405 



table; 3rd, radiates the heat conducted by the metal chimney, and 

 prevents the oil boiling. The burner is placed at one angle of the 

 reservoir to enable the flame to be placed very near the stage of the 

 microscope, which is exceedingly useful with some kinds of illumina- 

 tion, especially with reflected light, with the higher powers, and for 

 Powell and Lealand's super-stage condenser. 



The hole for filling the reservoir is placed at the diagonal corner 

 for convenience. The chimney is metal, with an ordinary 3 x 1 

 glass slip in front ; the diameter of the flame-chamber should not 

 exceed 1^ inch, and the grooves holding the glass slip should project 

 ^ inch from the flame-chamber ; the aperture should be only H inch 

 long; length of chimney should be 7 inches. Chimney should be 

 dead-black inside. This chimney serves four purposes: 1st, image 

 of flame is not distorted by stria* and specks common to ordinary 

 lamp chimneys ; 2nd, prevents reflexion from inner surface of 

 chimney, which causes a double image of flame ; 3rd, prevents 

 scattered light in room ; 4th, is not readily broken ; slips can be 

 easily replaced. 1 



By rotation of chimney either the edge or flat side of the flame 

 may be used. The bull's-eye is of Herschel's form, viz. a meniscus 

 and crossed convex; it is mounted on an arm which rotates e< ii 

 trally with the lamp flame. Unfortunately, as we have seen 

 (p. 332), there are errors in Sir J. Herschel's original calcula- 

 tion, and with these it has been copied by many opticians ; a lens, 

 it has been demonstrated, can be made on the Herschel formula, 

 as calculated by Mr. Nelson, having a minimum aberration. 

 The arm is slotted so that the bull's-eye may be focussed to the 

 flame ; it can be fixed by a clamping screw. The bull's-eye may 

 also be elevated or depressed and fixed by a clamping screw, not 

 shown in the illustration. The bull's-eye, having once been focussed, 

 is permanently clamped, and it is brought into or taken out of posi- 

 tion simply by rotation of the arm. There should be a groove in 

 the pillar with a steadying pin on the lamp to pi-event rotation 

 during elevation or depression. 



The form of the clamping screw is important ; it should be at the 

 upper part of the tube, and not at the lower, as shown in the figure. 

 This keeps the screw clean from oil, which always, to a greater or less 

 extent, exudes over paraffin lamps. The screw should be of that 

 form which closes a pinching ring round the rod, and not merely a 

 screw which screws on to the rod and bruises it. This lamp, if 

 made, as it should be, with a japanned tin reservoir and a cast- 

 iron tripod foot, is quite inexpensive. There is no justification 

 for a circular foot, except that it can be readily and well finished 

 in the lathe with better apparent results and less labour than other 

 forms. 



A small lamp is made by Messrs. R. and J. Beck. We illus- 

 trate it in. fig. 342. 



The base, A, consists of a heavy ring, into which a square brass 



1 It is very important to remove the metal chimney after use, or at least not to 

 leave it on when not in use, since the evaporating paraffin gathers round it and causes 

 undesirable scent when the lamp is again lit. The thinnest slips should be used. 



