Geometrical optics / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1913
Catalogue details

Licence: In copyright

Credit: Geometrical optics / by Archibald Stanley Percival. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    As BA is in the first focal plane, tan 6 = ~ and so the magnification M' in this case is ^n = — . - = 4 • ° tan 6 f o f Our lens of 4 inches focus, used in this way, would only give a magnification of j, or ^ = 2*5, but it would be less fatiguing to the eye. On comparing the magnifying power of a convex lens used in these two different ways, we see that in the first case it is 1 4- $), and in the second case it is ~, or - 1, ^. There- / / / fore, if d be less than /' the first method gives the higher magnification, and vice versa. Graphic Method for Lenses.—A method similar to that illustrated in Figs. 28 and 29 may be employed for finding the position and the relative size of the image formed by any lens. Tig. 28, when FF' is made equal to F'H, will represent the construction necessary for a convex lens whose first focal distance (/') is indicated by F'H, whereas Fig. 29 if inverted would give the construction for a concave lens when FF' is made equal to F'H, its focal distance. Such methods will, however, be seldom found to be of practical use, as we have pf» i f such simple formulae as q = — „, and - = -~ ready to * # P-f o /' - p J our hand for the solution of questions like these. Power of a Lens. Dioptres.—In a previous section we used the expression magnifying power, and we saw that it ivaried inversely as /'; we have seen, too, that strong lenses with strongly curved surfaces had short focal lengths. We are therefore quite ready to admit that y-, is an adequate measure of the power of a lens. The unit universally adopted is that of a lens of one metre focal length, which is called a dioptre, and is denoted by the symbol D. We know that a convex lens of 25 cm. focal length is four times stronger than one of 100 cm. focal length. This is very simply expressed by calling the former lens +4D and the latter