410 Major L. Darwin. On the Method of 



placed in an ordinary camera, which should be pointed at the sky; 

 if the ground glass is brought to the principal focus, the flare spoi- 

 ls then readily visible. 



For tests Nos. 9 to 16 an apparatus designed by myself, and which 

 I have called the " testing camera," is used. It is neither an ex- 

 pensive nor an elaborate contrivance, and there can be no doubt that 

 if more money had been expended a more perfect machine could have 

 been made. Until a system of this sort has been in regular use for 

 some time, and until it has stood the fire of criticism, experience shows, 

 we think, that the apparatus employed is apt to be little more than a 

 good working model of what it will become by future developments ; 

 but improvements would in this instance probably tend to increased 

 rapidity rather than to increased accuracy, for the results obtained 

 are now quite accurate enough for all practical purposes. Even now 

 alterations are under consideration, such as the substitution of a 

 sliding eye-piece on a graduated bar for the long sheet of ground 

 glass. For the above-mentioned reasons, and because much expen- 

 diture could not be justified until it was certain that lenses would 

 be sent for examination in considerable numbers, the Kew Committee 

 raised no objection to the somewhat make-shift appearance of the 

 apparatus. 



The general idea of the testing camera is extremely simple, but the 

 name perhaps is hardly a happy one, as there is no " camera " or 

 chamber about it. Except for the absence of bellows, it may be said 

 to consist of the essentials of an ordinary camera, which is capable of 

 being revolved horizontally about a vertical axis passing through the 

 lens ; though it must be confessed that this description gives no idea 

 of its appearance. The three-legged stool or bench, seen in fig. 1, 

 represents the legs of the camera, and fig. 2 shows the apparatus 

 that takes the place of the body ; G is the lens-holder, and LM the 

 ground glass, both of which are capable of independent movement 

 backwards and forwards on the hollow wooden beam DE, called the 

 " swinging beam." There is a conical brass cap or pivot, not shown 

 in the sketch, under the upper plank of the swinging beam, under- 

 neath where the lens-holder G is shown in the sketch. The whole 

 of the apparatus shown in fig. 2 is placed on the top of the 

 three-legged stool, the round-headed iron pin A passing loosely 

 through a hole in the lower plank of the swinging beam, and fitting 

 into the conical brass cap or pivot. The swinging beam, being 

 thus supported by the pin A and by the long arm BC of the stool, is 

 capable of being revolved round A as a centre. On the ground glass 

 is engraved a horizontal line, which is accurately divided into fiftieths 

 of an inch ; this line passes through the centre of the ground glass 

 (or through the point where the perpendicular from the lens-holder 

 cuts the glass), and is also parallel to BC, the top of the stool on 



