244« Df* Wollaston on double. Images 



When these appearances are first discernible, the variations 

 of density are so great, that the object to be looked at may be 

 in contact with the phial ; but, when the variations of density 

 become more gradual, and thereby the focus more distant, any 

 object so near is only elongated, and requires to be removed an 

 inch or two, to be seen inverted. 



Exper. 2. Over the surface of the water, in the same phial, 

 I next put about the same measure of rectified spirit of wine. 



At the stratum where the water and spirit united, the appear- 

 ances were the same; but, since the refractive power of spirit 

 exceeds that of water, the true place of the object was seen up- 

 permost ; the inverted and erect images are below. Fig. 6, B. 



When an oblique line der is viewed through any variable 

 medium so made, it appears bent into different forms, according 

 to its situation with respect to focal distance. 



If it be at the distance of the principal focus, one point of it 

 is dilated into a vertical line, as Im. Fig. 7, A. 



If beyond that focus, the portion lm is inclined backwards, 

 being an inverted image of dl; and mn is another image of the 

 same portion seen erect. Fig. 7, B. 



On this account, it becomes a convenient object for ascertain- 

 ing the state of any medium under examination. 



In these experiments, the appearances continue many hours, 

 even with spirit of wine ; with syrup, two or three days ; with 

 acid of vitriol, four or five ; with a solution of gum arabic, much 

 longer ; but, although their disposition to unite is so different, 

 the appearances produced are nevertheless the same in all. 



The refraction is greatest nearly in the plain of original con- 

 tact of the fluids, diminishing from thence both upwards and 

 downwards. The exact rate of diminution above or below this 



