12S On the Size lest adapted for Achromatic Glasses. 



body of the observer will sometimes agitate the floor enough ta 

 produce this effect. 



The atmosphere ahvays appears most diaphanous on those 

 evenings when there is least wind; and vision seems better, per- 

 haps, because the instrument is still. For this reason, and to 

 avoid currents of air passing before the glass, whenever the 

 weather will permit, let the telescope be taken out of doors ; for 

 it will never do its utmost unless it is placed on the ground, in 

 the open air. If the instrument has been kept in a room, the 

 temperature of which is much warmer than the open air, I usually 

 take off the cap of the object end, and take out the eye-piece, 

 and let the air pass through the tube for ten minutes ; and for 

 at least the same space of time we must carefully avoid all sti- 

 mulating and bright objects; so that the pupil may be in its 

 most expanded state. When the eve is thus prepffred, the sen- 

 sibility of the visual organ will be mucli increased. I have also 

 found it very advantageous to occasionally rest the eye for a few 

 minutes: this will restore its irritabilitv, which is soon exhausted 

 when stimulated by an intensely bright object : and when a light 

 is necessary to find an eye-piece, or rectify the instrument, to 

 prevent the adjustment of the eve being disturbed, I use a small 

 lantern, which gives a very faint light otily on one side, and that 

 may be made dark. 



For those who have not courage, or constitution, to brave the 

 inclemency of midnight frosts and damps, the most steady way 

 of supporting a telescope within doors, is by a clamp made to 

 fasten on the sashes when the top sash is put down : the object- 

 end of the telescope is then in the open air, and out of reach of 

 the undulating motion occasioned by looking through a medium 

 of atmosphere vvhich is undergoing a change of temperature, by 

 the cold air rushing into the warm room. By this contrivance 

 we have almost all the steadiness of being on the ground, with- 

 out being exposed to the cold, &:c. 



I must here endeavour to impress on the mind of my readers 

 another most imjjortant observation: when they have done using 

 tlie telescope, let the object-glass be taken out and laid in a dry 

 warm place, for a sulhcient time to evaporate the damp air, 

 which on dewy evenings too plentifully condenses on the object- 

 glass ; and however closely the lenses constituting the object- 

 glass are burnished into the brass cell, unless they are very care- 

 fully kept dry, the damp air will penetrate between the glasses, 

 and produce a sort of fog, or sometimes an arborescent vegeta- 

 tion like sea-weed, which I have seen spread all over the object- 

 glass. Unless these evils exist in a very extreme degree, ex- 

 perience has proved the only detriment they do to the perform- 



