38 OPTICAL PROJECTION 



With more light the heat was greatly increased, and for 

 some time it was found very difficult to prevent constant 

 cracking of the glasses in front of the flame-chamber. In the 

 Sciopticon, this glass was removed to a good distance and the 

 heat was less : but the triple lamps were constantly cracking, 

 until the introduction of what were for many years known as 

 the Newton patents. The first of these consisted in making 

 apertures in the front of the top of the flame-chamber, between 

 the glass and the chimney. So far from letting hot air out, 

 as many supposed, the strong draught drew cold air down, 

 and kept the glass cool enough to prevent cracking, except in 

 rare instances. Mr. Newton's later improvement was, however, 

 the most effectual, and consisted in heating the originally flat 

 squares of crown glass in iron tubes, till they took the curve 

 of the tube, and then annealing them. This allows the glass 

 to spring or curl with the heat, and I believe no such glass 

 has been known to crack, whereas no annealing was found to 

 absolutely prevent the breakage of flat glasses. 



To ensure a good light with these lamps, of whatever 

 make, the following directions must be attended to. The 

 wicks should in most lamps be put in new about seven inches 

 long. Each wick must be fed in and pressed a little, while 

 the milled head belonging to it is turned backwards, or to the 

 left, when the ratchet will catch the wick and draw it down 

 into place. As in the preceding case, the wicks must always 

 be carefully dried. Trim off as evenly as possible with sharp 

 scissors. The next thing is to unscrew the cap at the front 

 end of the cistern, and fill this with oil. I mention this, 

 because I have received actual proof that persons have 

 positively lit these lamps and expected them to burn with 

 no oil at all ! The quantity is generally about three quarters 

 of a pint for three wicks, and a pint for four ; but the cistern 

 should be filled nearly up. The common trash often sold as 

 1 paraffin oil ' should not be used ; much of it is not safe in any 

 lamp, and none of it gives a good light ; what are known as 



