On the Size lest adapted for Achromatic Glasses. 129 



ance of the glass is, that it does not transmit quite so much light: 

 and if the instrument be a very fine one, it is more advisable to 

 put UjJ with an ahnost imperceptible diminution of its brilliancy, 

 than run the risk of destroying the telescope— for the object- 

 glasses cannot be separated from each other, without perhaps ir- 

 reparably disordering the adjustment ; from the perfect harmony 

 of which, the instrument may, possibly, in a great measure derive 

 its superior excellence, from certain circumstances, which once 

 disturbed can never be restored — the acme of perfection being 

 always accidental. 



Moreover, it is equally indispensable, when we wish to discern 

 those delicate and minute objects which are the most interesting 

 and curioii$_ exhibitions our telescopes display to us, and with the 

 finest instruments are only discernible with the most favourable 

 circumstances, that we should be in a position of the greatest 

 ease : no cramp or painuil posture must distort the body, or ir- 

 ritate the mind ; the whole powers of which must be concentrated 

 in the eye : for such is the sympathy between the various organs 

 of the human body, that we may as well attempt to think ac- 

 curately on two subjects at the same time, as to use two senses 

 at the same moment: — each must be used alone. As our im- 

 mortal Shakespeare has observed of listening, with such profound 

 attention, that " each other sense was lost in that of hearing." 



TJie smallest achromatic that can be used with effect for astro- 

 nomical purposes is the three-and-a-half feet. These telescopes 

 were originally furnished with three object-glasses of three inches 

 and three-quarters diameter ; but they are now usually made 

 with two object-glasses of two iuches and three-quarters aper- 

 ture. With this telescope all the principal and most interesting 

 celestial phsenomena may be pleasantly observed : and indeed, 

 if exquisitely perfect, it will discover the minutest objects in the 

 heavens: and as there are more made of this than of the larger- 

 sized telescopes, it is proportionably more easy to obtain a good 

 one. In astronomical as well as in all other concerns, truth 

 and perfection are the first desiderata: our telescopes only de- 

 lude us, unless, like the juryman's oath, they display the truth, 

 and nothing but the truth. And, in future, I hope astronomical 

 amateurs will rather seek for perfect telescopes than large ones; 

 for, as the pupil of the eye contracts and dilates pro re-nald, 

 bright ol)jects would often be better seen by reducing the aper- 

 ture, than by loading the telescope with magnifying power to 

 &ave the eye from being drowned in light. 



Vol.46. No. 208. ^m^mjUSIS. I XXVII. 0» 



