270 Work for the Telescope. 



for accurate observations, instruments are mounted on the most 

 solid piers of stone or brickwork, entirely insulated from the 

 tremors of the surrounding floor; and hence the disadvantage 

 of keeping the same boards under our telescope and ourselves. 

 On all accounts, therefore, an out-door situation is best; and 

 we may be assured, from the almost universal experience of 

 astronomers, that the much and very unjustly calumniated 

 " night air" will work us no kind of harm. Of course, we shall 

 use the ordinary precautions against catching cold; and our 

 instrument will require equal attention, for object-glasses 

 have a very inconvenient facility in catching dew. If brought 

 uncovered suddenly into air of a very different temperature, this 

 is pretty sure to happen, just as it does to a glass of cold water 

 brought into a heated room ; and in a dewy or frosty evening 

 it will take place from radiation. The best mode of obviating 

 this is to take off and ultimately replace the cap of the object- 

 glass in the air in which it is used, and to provide it with a 

 " dew-cap," a cylinder of pasteboard, light thin wood from a 

 hat-box, or bright tin blackened in the interior, two or three 

 times the length of its own diameter. On the removal of the 

 brass cap, this should immediately be substituted for it, and 

 remain till the last ; and when the brass cap is about to be 

 replaced, it will be advantageous to warm it previously. Where 

 there is a finder it should be provided with a dew-cap also. If 

 dew should have accidentally settled upon an object-glass, it is 

 a good plan to unscrew it, and hold it before the fire ; this will 

 diminish the necessity of frequent wiping, which should be 

 avoided, as the polish is very liable to injury ; but the removing 

 and replacing of an object-glass of any size demands a careful 

 hand. After all, little specks will be formed in time, and occa- 

 sional cleaning will be required; this may be safely done by 

 dusting the glass with a camel's hair pencil, breathing upon it 

 very slightly, and wiping it gently with very fine wash leather, 

 or a real silk handkerchief, both of which should be free from grit 

 and dust, and kept for the purpose. Foreign glass will some- 

 times become discoloured even from the contact of loose par- 

 ticles which may have long lain upon it ; no amount of friction 

 will remove these stains which will not endanger the polish. If 

 objects appear dim and foggy on a clear night, damp has, in all 

 probability, settled on the surface of the eye-lens ; this may be 

 immediately dissipated by taking out the eye-piece, and hold- 

 ing it before the fire : the eye-piece of the finder is especially 

 subject to this annoyance, as it is less frequently protected by 

 the application of the eye : if the fire is not conveniently acces- 

 sible, a bit of wash-leather may be kept at hand. 



As no positive or Ramsden eye-piece (which has the plane 

 surface of each of its lenses outwards) is achromatic, and even 



