336 Professor J. D. Everett on a 



When the lamp or mirror has once been adjusted to throw 

 its light in the proper direction, it should not be disturbed, 

 as all necessary adjustments can be better made by moving 

 the instrument. 



The lens and screen may conveniently be mounted first, 

 and the adjustments made so that the light collected by the 

 lens falls on the screen as a horizontal beam. The cross is 

 then to be mounted in such a position that a bright patch 

 corresponding to the square hole is seen on the screen, sur- 

 rounded by the shadow of the card. The frame must now be 

 extended or compressed till the image of the cross appears in 

 the bright patch ; and the lens, object, and screen should then 

 be carefully set square by hand before the final adjustment. 

 If the vertical and horizontal lines of the cross do not focus 

 simultaneously, it is a sign that the lens needs setting square. 



The focussing having been completed, the distance of the 

 object from the image is to be measured and divided by four. 

 This will give the focal length ; and the calculation can be 

 checked by measuring one or more of the four equal parts 

 into which the distance is divided by alternate pins. Owing 

 to slight play in some of the joints, or other mechanical 

 imperfections, the theoretically equal distances may exhibit 

 sensible differences, especially when the frame is nearly 

 closed up ; but the method of observation is so well con- 

 ditioned that these inequalities do not practically affect the 

 correctness of the result. 



In fact, if the distances of the lens from the object and 



image, instead of being exactly equal, are a + x and a — a?,. 



a* — ® 2 

 the true focal length is — 9——? and in taking it to be one 



fourth of the whole distance we are simply neglecting x 2 in 

 comparison with a 2 . Suppose the two distances a + x and 

 a — x to measure 20-J and 19-J inches, which is a larger 

 inequality than is likely to occur, the ratio of x 9 - to a 2 is 1 to 

 1600 ; and this error is negligible, in view of the fact that 

 the doubt as to when the image is sharpest involves an un- 

 certainty in the focal length to the extent usually of more 

 than one per cent. 



When the focal length does not exceed 10 or 12 inches, 

 the instrument may be supported with the two hands and 

 pointed towards a gas-flame, which need not be at the same 

 level, but may be at any height. A fairly good measure- 

 ment can thus be made by one person, if there is opportunity 

 for setting the instrument down on a table or floor when the 

 lens needs setting square, and when the final measurement 

 of distance is to be made. The friction at the joints of the 



