MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 9 



Terv irregular, so that, for example, it would be quite impossible to alter 

 the position of the eyepiece to a jAace suitable for seeing the new image. 



The twinkling of stars also jDroduces marked changes in their colors. 

 A bright star, near the horizon, will show these changes to the naked 

 eye. In the telescope, and especially in the spectroscope, they are 

 very obvious, and at times quite fatal to measurements. They are all 

 due to changes in the temperature and arrangement of the strata of the 

 atmosphere, and are only absent when these strata are arranged con- 

 centrically in parallel layers. 



The deformations of star-images and of star-spectra have been studied 

 by Arago, Secchi, Dufour, Montigny, and others, to whose memoirs 

 reference is made in passing. The object of the foregoing paragraphs 

 is simply to describe the general effects of bad definition — bad seeing 

 — due to inequality in the distribution of temperature in the atmos- 

 pheric strata in the neighborhood of the observer. These effects are 

 more apparent the larger the aperture of the telescope employed and 

 the greater its focal length. 



More bundles of rays, coming from more directions, fall upon a large 

 object-glass than upon a small one. The changes in focal length due 

 to " air-lenses " are expressed in per cent of the focal length itself, and 

 hence the absolute displacement of the disturbed image, in inches, is 

 greater when long telescopes are employed. It is for these reasons 

 that it is especially necessary to select suitable sites for the emplace- 

 ment of the large telescopes of modern times. The study of the con- 

 ditions of seeing at mountain observatories thus becomes of capital 

 importance. 



In a general way it may be said that the air-strata over an extensive 

 plain arrange themselves horizontally. On a mountain peak, however, 

 they are necessarily curved, except when a complete calm prevails, 

 and when, also, the temperature-conditions are the same for considera- 

 ble distances above and below the peak, if the whole air is clear, or 

 from a considerable distance above the peak down to the upper surface 

 of an extensive fog-layer itself lying some distance below the summit. 

 The last arrangement describes the usual summer conditions at night 

 on Mt. Hamilton. A capital merit of our climate is that the vision 

 usually continues good during the entire night if it is good at the 

 beginning. 



An important advantage to be sought for in the site of an astro- 

 nomical observatory is the continued clear weather. Much time is 

 spent in preparing for observations ; and this is time lost if the observa- 

 tions are prevented by clouds or fogs. If one is sure of good weather, 

 a programme of observation may be made weeks beforehand, and 

 carried out to the letter at the appropriate time. 



