ACCESSOEY IKSTRUMENTS. 153 



Particular attention ought to be paid to the size and shape 

 of the chimney. In passing a chimney over the flame into its 

 place, it may be seen that there is one point where the flame 

 is at the brightest ; this point should be noticed, and if the 

 glass when in its place does not keep the light at the same 

 intensity, then the contracted part or shoulder of the chimney 

 is either too high or too low for the surface of the wick; a few 

 experiments wiU soon settle this point. If the maximum of 

 light be obtained before the chimney comes into its proper 

 place, then the contracted part or shoulder of the glass is not 

 high enough ; if, on the contrary, the maximum of light be not 

 obtained, then the shoulder of the chimney is too high ; hence 

 the necessity of having a gallery that can be adjusted to 

 chimneys of diiferent heights, as recommended by Mr. 

 Gwilt.* Generally speaking, the shoulder of the chimney 

 should be on a level with the top of the wick, and its diameter 

 at that part should not be more than two-thirds of an inch 

 greater than that of the outside of the wick. Those chimneys, 

 provided either with a disc of metal or talc, or which are con- 

 tracted just above the wick, as seen in fig. 105, appear to 

 answer the best, as with them the most intense %ht is pro- 

 duced. Chimneys have lately been made of a light blue or 

 neutral tint glass, which answer their purpose exceedingly 

 well, as they completely destroy the yellow colour of the 

 flame, and render it beautifully white. If a lamp having one 

 of these chimneys be placed by the side of another having a 

 chimney of the ordinary kind, the diflTerence between the two 

 will be very striking. 



Chimney Shade. — This piece of apparatus is described by 

 Mr. Holland, in the forty-ninth volume of the Transactions of 

 the Society of Arts. It consists of a tube of brass, a little 

 longer and broader than the chimney of the lamp, having on 

 one side a brass plate with an aperture three-quarters of an 

 inch in diameter, that can be moved up or down in front of 

 the flame of the lamp by a rack and pinion. The tube cuts 

 off all the light from the room, except that which can pass 

 through the aperture above described, and its use is that of 

 * Microscopical Journcd, vol. i., p. 56. 



