Allison. — The Star Test for Telescopic Mirrors. 499 



mirror. A good hint can be obtained from Cooke,* and after inaking tlie 

 necessary alterations I have no hesitation in quoting from him :■ — ■ 



" If on racking towards the mirror it is found that the central rings 

 look feeble while the edge rings, and especially the outer one, look massive 

 and luminous, while on racking out of focus away from the mirror the 

 central rings look relatively brighter and the outer rings look weak in com- 

 parison to what they appeared when within focus, then the inference is that 

 the edge rays fall short or come to a focus at a point nearer to the mirror 

 than the focus for the central rays, or, in other words, there is positive 

 aberration. 



" If, on the other hand, the central rings when inside focus look about 

 as luminous or even more so than the outer ring which is thin and weak, 

 while on racking outside focus the complementary effect of a massive 

 and luminous outside ring enclosing comparatively feeble central rings is 

 observed, then the inference is that the edge rays come to focus at a 

 point farther from the mirror than the focus for the central rays, and this 

 fault is negative aberration." 



It will thus be seen that the general disc of light gives a general idea of 

 the figure. The black spot in the centre being larger on one side or the 

 other shows at once whether the mirror is imder- or over-corrected ; and 

 the outside ring of light viewed at equal distances inside and outside the 

 focus, ^\^ith a succession of stops getting smaller and smaller, shows at once 

 the figure of each successive zone, and whether polished too long or too 

 short in focus ; or, if absolutely equal, quite correct. 



When finally silvered and tested this way it gives at once the general 

 figure and the individual zones. If there are zones in the mirror they are 

 easily seen in the expanded discs of light, as these zones appear brighter or 

 darker than the normal disc. 



If the edge is badly turned down there will probably be a ring-system 

 outside the focus, with a heavy outside ring, while inside of the focal point 

 the disc of light will be faint on its margin, with a hairy and confused edge. 

 It is useful to have a number of stops made reducing the aperture, and 

 then to compare the outside ring as the aperture gets smaller. This outside 

 ring should be the best-defined ring on the disc, and should be exactly 

 equally bright inside and outside of focus at equal distances. If this is 

 not the case there is a zone at the edge of the circle to which the mirror is 

 stopped down, and it can be ascertained in manner previously mentioned 

 whether it is polished too deep or not deep enough. 



Another defect easily detected by the out-of-focus method is astig- 

 matism. Astigmatism may be caused either by the large mirror, the eye- 

 piece, or the eye of the observer ; or the small flat may, if not flat, show a 

 similar defect. The appearance of the out-of-focus image will be an oval 

 instead of a circular disc. 



If it is caused by a defective eye-piece the long diameter of the oval 

 image will rotate as the eye-piece rotates. If it is caused by the eye of the 

 observer it is only necessary to move the head a quarter of a circle round 

 the eye-piece to see if the oval rotates. There remains the chance of the 

 flat being in fault. If the large mirror is turned 90° and the oval image 

 does not rotate the flat is in fault. If the oval rotates with the large mirror 

 the fault lies with the mirror — that is, it is astigmatic : one diameter is 

 ground and polished slightly flatter than the other diameter. This method 

 of testing is particularly suited to finally testing large mirrors after the}' 



* " Telescopic Objectives." London : De Little and Sons, 1896. 



