TEL 



the least defect in the figure or adjustment of the object g'lass is imme- 

 diately seen by the star not appearing round, but surrounded by false 

 lights and luminous accompaniments. For a test of the perfection of a 

 telescope as to its light and distinctness, the pole star is as proper as any, 

 as the small accompanying star is not visible except in a very perfect 

 instrument. The examination of a bright object on a dark ground, as a 

 card by daylight or Jupiter by night, with high magnifying powers, af- 

 fords the severest test of the perfect achromaticity of a telescope, by the 

 production of green and purple borders about their edges in the con- 

 trary case. ( Kitchiner. Mem. Astr. Soc.J 



On the evenings and situations favourable or otherwise to astronomical 

 observations. 



The rule upon which almost all the rest are founded is that an uniform 

 temperature is necessary for the proper performance of a telescope. 

 Upon this principle the following facts, the results of long experience, 

 may be satisfactorily explained. 



(1) A frost after mild weather, and a thaw after frost, will derange the 

 telescope, till either the frost or mild weather are sufficiently settled. 

 (2) No telescope just brought out of a warm room can act properly. (3) 

 No delicate observation with high powers can be made when looking 

 through a door, window, or slit, in the roof of on observatory ; even a 

 confined place in the open air is detrimental. (4) Windy weather is un- 

 favourable. (5) Stars seen over the roof of a house, when very near, are 

 not distinct, being disturbed probably by warm exhalations from the 

 roof. (6) Dry air is unfavourable; but those evenings wherein the air 

 is saturated with moisture, so as to drop down the tube of the telescope, 

 are particularly favourable to distinct vision. 



Upon the whole Dr Herschel observes that to use the highest magni- 

 fying powers to the greatest advantage, the air must be very clear, the 

 moon absent, no tAvilight, no haziness, no violent wind, no sudden 

 change of temperature ; under all these circumstances a year that will 

 afford 100 hours must be called a very productive one. (Herschel t Phil. 

 Trans.) 



Rules necessary to be observed for examining delicate objects with suc- 

 cess. 



(1) If the telescope has been kept in a warm room, the cap of the ob- 

 ject end should be taken off, the eye piece taken out, and the air suffer- 

 ed to pass through the tube for ten minutes, that it may acquire the tem- 

 perature of the open air. (Kitchiner.) 



(2) The observer should in like manner be exposed in the open air for 

 15 oi % 20 minutes, and the eye carefully kept from all stvnulatin^ aud 



