660 PHILOSOPHICAL TRANSACTIONS. [aNNO 17q6. 



base, near Old Sarum, the top of the flag-staff at Beacon Hill was elevated 

 38' 42". The axis of the telescope at Beacon Hill was 15 inches above, and 

 the top of the flag-staff 9 1 inches above the point where the mensuration began. 

 Near Old Sarum it was 28 inches higher, and the top of that flag-staff 95 inches 

 above where the base terminated. This end is 429-48 feet lower than the other. 

 Lastly, the value of the base is 6' of a degree, very nearly. 



END OP THE EIGHTY-FIFTH VOLUME OF THE ORIGINAL. 



/. The Croonian Lecture on Muscular Motion. By Everard Home, Esq. F. R. S. 



Juno 17 g6. Fol.86. p. 1. 



In the Croonian Lecture which I had the honour of laying before the r. s. 

 last year, I endeavoured to prove, that the adjustment of the eye to different 

 distances could take place independent of the crystalline lens ; and that when 

 this was the case, it appeared to arise from a change in the curvature of the 

 cornea. I propose in the present lecture to prosecute the inquiry ; and it will 

 be found in this, as well as in the former, that I have received the most essen- 

 tial assistance from Mr. Ramsden, who continues to interest himself in the 

 investigation, and has made all the optical experiments. As this was a new 

 mode of explaining the adjustment of the eye, and differed from the theories 

 that have been previously formed on the subject, it was thought right to consider 

 it with caution, to pay attention to all the objections that could be made to it, 

 and to put it to the test of such experiments as appeared likely to refute or con- 

 firm our former observations. 



It readily suggested itself, that if the convexity of the cornea was increased to 

 a certain degree, it could be measured by means of an image reflected from its 

 surface, and viewed in an achromatic microscope, with a divided eye-glass mi- 

 crometer. To ascertain whether the quantity of increase of the convexity of 

 the cornea, in the adjustment of the eye, could in this way be ascertained, the 

 following experiments were contrived, and made by Mr. Ramsden. Our former 

 experiments had sufficiently proved the unsteadiness of the human eye; the first 

 trials on the present occasion were therefore made on convex mirrors, as these 

 artificial corneas could be more readily managed, and such previous experiments 

 would enable us to apply the same instruments with more facility to the eye 

 itself. 



Two convex mirrors, one -^ of an inch focus, the other -j^, had their flat 

 surfaces made rough, and blacked, to prevent an image being seen from both 

 surfaces, which was found to be the case when this precaution was omitted. 

 One of these mirrors was stuck on a piece of wood directly opposite to a win- 



