LOW-POWER WORK 29 



given regarding the method of supporting the Hme and the condenser. As the hght 

 is to be raised vertically up or down to get the required obliquity, so the lime must 

 also be tilted with the condenser to allow the issuing rays to fall upon the tube fixed 

 beneath in its bath. We have found that a convenient arrangement for this tilting 

 is afforded by fixing the tray of the jet upon a piece of wood which is hinged at its 

 ends to another piece, as shown in Fig. 17. A simple wedge is now all that is 

 required to raise or lower the end of the upper board, so as to make the rays tilt 

 enough to impinge on the tube. To make our meaning clear, we may say there are 

 really two movements required ; one to raise the light above the object, the other to 



Fig. 17 Fig. 18 



tilt the condenser sufficiently down to bring the rays obliquely on to it. The 

 arrangement is shown in Fig. 17. 



In takmg this class of photographs it must be distinctly understood there is no 

 hard-and-fast rule ; each position that we have explained must be tried by direct 

 experiment until the results required are obtained. Occasionally ti-oubles may arise 

 from bubbles forming in the water-bath while the apparatus is being got in order. 

 These must be carefully looked for at the last moment before taking the photograph, 

 as it is annoying to find afterwards that their presence has spoilt the final effect. A 

 pencil or stirring-rod is useful for this purpose, and should always be handy. 



If water be used, and not glycerin, it rapidly becomes hot if much time is spent 

 arranging the details for taking the photograph ; this, too, should be carefully 

 guarded against, as with some tubes the slightest heat will melt the gelatin and 



