of the Heavenly Bodies. 19 



the horizontal diameter of the sun, and established so distinctly 

 by the plain perceptions of sense, that nothing but that ingenious 

 learning, which in every age has puzzled the plainest things, 

 could have induced a doubt respecting what was actually seen, 

 although the cause was not understood. 



Riccioli accounts for the differences of opinion and measure- 

 ment of others, by supposing that, by the adopted modes of 

 measurement, which he states, differing from his, the external 

 light of the limb of the object is intercepted, and its size reduced. 



I have endeavoured variously to account for these continued 

 errors of the acutest observers, by supposing that taking it for 

 granted that all changes of place, or refractions, as they called 

 them, were made in vertical circles, which is not correct, and 

 that all measurement of any but the vertical diameter of the 

 luminaries was unnecessary, upon finding this to be what was 

 expected, they abandoned all further observations of horizontal 

 diameters, not perhaps so conveniently measured as in the 

 vertical line, or made them without sufficient care and attention. 



The rainbow has been referred to, as exhibiting near the 

 horizon, a considerable increase of breadth, upon the same 

 principles which produce the apparent enlargement of the 

 horizontal sun and moon, and thus confirming them. But one 

 error is here adduced to support another. The cases are en- 

 tirely different. The rainbow is seen under very different 

 circumstances, and at short distances, which allow neither the 

 imagination to act nor the atmosphere. Of the falling drops 

 of rain, the increased sizes near the horizon, extend the spaces 

 of formation of the radiants which they reflect, increase their 

 number and divergence, and thus enlarge the dimensions of the 

 bow which they form. I have seen the primary rainbow com- 

 pletely formed, and at the same time the following appearances 

 exhibited : In the thin vapour of the cloud on high, was formed 

 a narrow principal bow, attended with three or four inflected 

 orders of colours, which are known to depend upon the small 

 sizes of the drops. By degrees, lower down, these accompanying 

 bows disappeared by uniting into the single primary, whose 

 dimensions in the horizon became dilated into more than double 



C2 



