2o6 



NATURE 



[December 31, 1891 



(2) This adjustment is technically called " squaring 

 on," and is usually provided for in telescopes over 3 

 inches in aperture. If the optic axis of the objective 

 does not pass through the centre of the eye-piece, the 

 diffraction rings which are seen when the star is out 

 of focus, will appear oval, and the focused image will be 

 fan-shaped (p. 8). This follows from the fact that we are 

 dealing with an oblique section of the cones of rays from 

 the object-glass, and the rings will be most expanded and 

 dimmest on the side which is furthest from the object- 

 glass. 



Fig. 2, a, represents the rings seen when the star is out 

 of focus in the case of an objective seriously out of 

 square, and Fig. 2, h, the corresponding focused image. 



(3) Ordinary objectives should be so corrected that all 

 the rays between C and F of the spectrum are brought 

 to a common focus, these being the rays to which the 



Zonal aberration, in which different parts of the ob- 

 jective have slightly different foci, modify the ring 

 systems in a very remarkable fashion. In this case, the 

 rings will not gradually diniinish in intensity, but will vary 

 according to the degree of imperfection. Figs. 4, rt;,and 4, b, 

 show a good example of this, being sections taken within 

 and outside the focus respectively. 



(5) The effect of astigmatism in the objective will be 



Fig. 2. — Appearances observed when the objective is out of square. 



retina is most sensitive. When this is done the objective 

 is " over-corrected," and the rays less refrangible than C 

 are brought to a shorter focus, while those more re- 

 frangible than F are focused at a greater distance outside. 

 Hence, supposing a white star like Vega is observed, the 

 focused image should be surrounded by an almost im- 

 perceptible blue fringe. A little way within the focus the 

 image should show a reddish nucleus, and outside the 

 focus a bluish centre should be observed. The effect of 

 the colour of the star observed must be carefully guarded 

 against (p. 16), and an eye-piece of sufficient power should 

 be employed, for the reason already stated. 



A good method of testing for achromatism is to focus 

 the image of a star on the slit of a spectroscope. If the 

 image be perfectly achromatic, as in the case of a re- 

 flector, the spectrum seen will be a line of uniform thick- 

 ness. Any departure from this will be indicated by local 



to'produce ellipticity in the rings, a very decided example 

 of which is shown in Fig. 5. 



-; Fig. 5, a, is a section taken within the focus, and Fig. 5, b, 

 a section taken at the same distance outside. The com- 

 bined effects of an astigmatic objective and an astigmatic 

 eye may obviously be very variable. 



Other causes may also operate in the modification of 



FliJ. 5. — Elliptic ringsproduced by astigmatism. 



i the diffraction rings, and some good examples are given 

 The effects of the flexure of a lens supported on three 

 points, for instance, are admirably shown in Fig. 6, a. 

 Those who have seen photographs in which bright stars 

 appear with their conventional rays will now have no 

 difSculty in understanding their origin. They depend 



i simply upon the distortion of the lens or mirror with which 



-Appearances due to spherical aberration. 



widenings of the spectrum. With an ordinary objective, 

 the spectrum should be a narrow line from C to F, 

 widening out at each end. 



(4) The absence of proper correction for spherical 

 aberration produces very interesting features in the 

 diffraction rings, some of which are admirably shown by 

 the diagrams which are reproduced in Figs. 3, a, and 3, b. 

 These represent sections of the cone of rays within and 

 outside the focus respectively in the case of a lens in 

 which there is positive alaerration — that is, in which the 

 rays from the outer parts of the object-glass coma to a 

 shorter focus than the central rays. In the first figure the 

 concentration of light in the outer ring is the chief 

 characteristic, while in the second the central ring is 

 relatively brighter. 



NO. I 157, VOL. 45I 



Fig. 6. — Diagrams showing effects due to (a) flexure; {b and t) strain in 

 cell ; (d) veins in objective. 



the photograph was taken, and the number of rays will 

 correspond to the number of points of support. 



An objective which is strained in its cell, but properly 

 squared on, may produce a distortion of the rings similar 

 to that shown in Fig. 6, b. A more violent case of 

 mechanical strain is shown in Fig. 6, c, and the presence 

 of veins of unequal refractive power may affect the rings 

 somewhat in the manner shown in Fig. 6,d. 



A, Fowler. 



