'Mr. herschel’s Account 
table x* Meafures of the Comet’s diameter 
Days. 
// /// 
Powers. 
March 17 
2 53 
932. 460. 
19 
2 59 
932. 460. 
21 
3 38 
460. 
28 
4 7 
1 thefe meafures agree to q"'« 
— 
3 58 
227 J 0 y 
29 
4 7 
227 rather too fmall a meafure. 
4 25 
227 feems right. 
April 2 
4 25 
227 
6 
4 53 
227 
15 
5 11 
227 
5 20 
227 very good; not liable, to halt a fecond of error. 
18 
5 2 , 
227 true to -I2'" or .18'" at rnoft. 
Having measured the diameter of the Comet, with fuch 
high power as 932 and 460, it may not be a mil's to make one 
observation on this fubjeft, left it fhould be mifapprehended 
that I pretend to a diftind power- of fuch magnitude upon all 
celeftial objects in general. By experience I have found, that 
the aberration or indiftinftnefs occalioned by magnifying much, 
provided the object be ftill left fufficiently diftind, is rather to 
be put up with, than the power to be reduced, when the angles 
to be meafured are extremely fmall. The reafon of this may, 
perhaps, be that a fmall error of judgement, to which we are 
always liable, is of great confequence with a low power, as 
bearing a confiderable proportion to the diameter of the objedt j 
* There are feveral optical deceptions which may affe£t the meafures of obj-e&s 
that fubtend extremely fmall angles. Thus I have found, by experience, that a 
very fmall object will appear fomething lefs in a telefcdpe when we fee it firft than 
when we become familiar with it. There is alfo a defleftion of light upon the 
wires when they are nearly fliut; but as none of thefe deceptions are well 
enough underflood to apply a correction, 1 leave them affedted with them. 
whereas 
