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this mode of illumination in this inftriinient, he being juftly 

 cautious left the heat of the candle might have any effed on the 

 axis ; but feveral hours carefully employed by him and me in 

 critical experiments, fatisfied him fully that his fears were ground- 

 lefs, and I can now by experience of the inftrument pronounce 

 them fo. 



This mode of illumination has many advantages. Firft, the 

 objed is not rendered tremulous by the vapor and flame of the 

 candle decompofing the air at the objed; glafs. Secondly, no 

 part of the light afforded by the objed glafs is loft. Thirdly, you 

 can diminifh your aperture at pleafure without the trouble of 

 altering your illuminator ; whereas if the annular one be applied 

 a different elliptic ring muft be provided for every diminution of 

 aperture ; if the folid illuminator be ufed, you cannot diminifti 

 the aperture very much, and preferve the valuable central rays*. 

 Another ufe of this method has occurred to me fince I began to 

 apply it, which is this — by accurately dividing the ftem of the 

 glafs-frame, and removing the green glafs, we might more cer- 

 tainly determine the different magnitude of the ftars, by noting 

 the quantity of light neceffary to efface them. 



I NOW come to the principal improvement in this inftrument; 

 an invention that does high honour to Mr. Ramfden, and is a 

 moft valuable acquifition to all aftronomical inftruments where 

 plumb lines are introduced. 



• The advantages of a great diminution of aperture on particular occafions I 

 Ihall mention in a future paper. 



The 



