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VI. Note on the Focometry of Lens- Combinations. 

 By Prof. A. Anderson *. 



1AM much obliged to Mr. Robert E. Baynes, M.A., for 

 directing attention to what is undoubtedly a defect in 

 my paper which appeared in the Philosophical Magazine of 

 January last — the example at the end. The combination con- 

 sisted of two concave lenses, but I cannot now submit it to 

 further measurement, as I kept no note of the particular 

 lenses used or of their distance apart. The measurements 

 were very rough and rapidly made, and the nodal slide 1 

 used is far from being an accurate instrument. It was made 

 in the laboratory workshop for demonstration purposes. 



The principle of the method is the same as that of the 

 method of Abbe, and whatever defects the one method pos- 

 sesses the other has too. The distinctive characteristic of the 

 method I proposed is the simple way in which the magni- 

 fication is measured, which is suitable not only for converging 

 combinations, but also for diverging combinations where an 

 auxiliary convex lens must be used. I admit, however, that 

 if its claim to recognition rested solely on the example given 

 it would not have much to recommend it. 



I have measured with some care another diverging com- 

 bination, but I must point out that the results are much 

 more a test of the capability of my rude nodal slide than of 

 the method. 



The combination consists of two concave lenses A and B. 

 The focal length A is 20*34 cm., and that of B 30*91 cm., 

 and their distance apart is 12*45 cm. By calculation it will 

 be found that/=9*87 cm., BH 2 = 6*04 cm., BH, = 8*47 cm., 

 and H 2 H 1 = 2*43 cm. The light falls first on A, and an 

 auxiliary lens is placed to the left of B to form an image on 

 a screen. It is this image which is not displaced when the 

 combination is rotated about 0. The position of an object 

 whose image is formed on the screen by the lens alone gives 

 the position of the image formed by the combination. The 

 measurements gave OV 1 or .r 1 = 142, OP 2 or ^ = 9*4, OPi' or 

 4*2 = 29*1, OP 2 7 or ^2 = 8*3, and d, the distance through which 

 the combination was moved to the right, 113*8. We thus 

 have 



113*8 _ 142 _ 29-1 

 / '" 9*4 8*3 ' 



giving f= 9*81 cm., which does not differ much from 9*87 cm., 

 the calculated focal length. 



* Communicated by the Author. 



