Licence: In copyright
Credit: General and practical optics / by Lionel Laurance. Source: Wellcome Collection.
Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)
336/390 page 326
![possible ; then L F is the principal focal length. If the lens is weak, or very strong, indirect focalisation (q.v.) is needed. Instead of the collimator, any distant bright source as a window or artificial light, can be employed. As F is short, the image is small, sharp, and bright, so that this method serves very well for fairly strong Cx. sphericals, but is uncertain for weak or very strong ones. To focalise a periscopic Cx. lens the distance of the optical centre from the lens must be obtained. The distance from the lens to the screen should be taken first with the one face, and then with the other, turned towards the source of light. The mean of the two measured distances is the true focal length. With ordinary periscopic spectacle lenses, the distance of F, from the lens itself, is sufficiently exact in practice. Indirect Focalisation.—If the lens is weak and therefore of long F the image on the screen is large and indistinct, and the exact principal focal distance is difficult to determine. If F is very short, the exact distance also becomes hard to determine with accuracy. For these we employ indirect focalisation. The procedure is to combine, with the unknoiun lens, another Fig. 318. hiotu7i lens ; find the focal length of the combination, and then deduct from the Ijower of the combination that of the unknown lens ; thus ¥ = l-¥, °' I>2 = D-I>i where F and D are, respectively, the focal length and the power of the two lenses combined, F^ and those of the added lens, and Fg and Dg are those of the unknown lens. The approximate power to be added can be found experimentally, and it is better to divide this power between a pair of lenses, placing one on either side of the unknown lens. A very strong Cx. lens should be combined with a Cc. lens of sufficient power to lengthen the focal distance to a reasonable extent. For instance, it is difficult to determine whether F = 2 in. or 2^ in. ; but if the lens ])e focalised with, say, a Sin. Cc, the diff'erence between the one and the other is much more marked, it being then about 1 in. Thus if F = 9 in. and Fi = - 3 in. 1/F2= 1/9 - (- 1/3) = 4/9 ; the lens is 2J in. Cx. or D2 = 4-5-(-13)=l7-5, or say+18. If F = 6 in. and F^ = - 3 in., then l/F2=l/6-(-l/3) = 3/6; the lens is 2 in. Cx. or 6-5-(-13) = 19-5, or say+ 20.](https://iiif.wellcomecollection.org/image/b21287946_0336.jp2/full/800%2C/0/default.jpg)
