236 



HORIZONTAL MOON, 



The image Exp. 1. I took a clear double convex lens fupported on % 



StbrlaV'"'^^"^* and placed the fame oppofile an Argand's lamp; fo 

 aperture is the that a good image of the flame was formed upon a paper fixed 

 iamt as againft the wall of the room. From the flame to the lens the 



diftance was 49|- inches and from the lens to the image it was 

 88 inches. The horizontal breadth of the image was repeat- 

 edly meafured = 1.58 inches and to a greater degree of pre- 

 cifion than one hundredth of an inch, 

 with a con- ^^P* 2. The lamp and lens were fuffered to remain unaltered 



Crafted aperture, j'n the fituations they poflefled in the former experiment; but 

 a fcreen was placed clofe to the lens, having a circular hole 

 of one inch diameter concentric with the lens iifelf. The 

 image was then fainter but very diftinct, and meafured exa6tly, 

 the fame as before. 



In both thefe experiments the papers on the wall were fe- 

 veral times changed to prevent any deception from the appear- 

 ance of former meafures; and the nieafures were taken along 

 the fame horizontal line by the help of a line on the wall. 



Hence it was feen that a diminution of the pupil or aperture, 



to admit lefs than one thirty-eighth part of the light, made no 



difference in the image. 



The experiments Exp. 3. I took a good achromatic objed glafs of Dollond, 



repeated with anj^^^j^^ ^^ aperture of 1.9 inch and placed it at the diftance of 



achromatic lens. !. , , , . , . , 



52| inches from the lamp; when it gave a good image on the 



wall at the diftance of 70| inches from the lens. The hori- 

 zontal breadth of the image was 1.2 inches. 



Exp, 4-. When the experiment was repeated with no other 

 change than that a fcreen with a central hole of 0.3 inches 

 was placed before the lens, the image proved 1 .2 inches as be- 

 fore. 



The two laft experiments were made with the fame care 



and precautions as the former. They (hew that no diminution 



of the image takes place when the pupil isdiminithed toadmit 



lefs than one fortieth part of the light. 



Probability that I may not conclude. Sir, without making fome obfervations 



Mr. W. was ^^ j|jg (\\reQ, con trad id ion of my experimental refults to thofe 



deceived, and ,-rT n . i,^ t r ■, , ■ • • i 



how. ot Mr. Walker s. May 1 alcribe to him any thmg more than 



inaccuracy.? Afluredly I do not. Neverthelefs, if he raea-, 

 fured fo uncertain a thing as the length of the flame of a candle 

 without being afliired by repeated experiments that its perma- 

 nency was entitled to fome confidence -, if he held his fcreen 



nearer 



