ocular Spedlra of Light and Colours* 247 
the eye, the fpe&rum will appear magnified in proportion to 
the diftance. 
I was furprifed, and agreeably amufed, with the following 
experiment, I covered a paper about four inches f mare with 
yellow, and with a pen filled with a blue colour wrote upon 
the middle of it the word BANKS in capitals as in fig. 5. and, 
fitting with my back to the fun, fixed my eyes for a minute 
exadtly on the center of the letter N in the middle of the 
word ; after clofing my eyes, and (hading them fomewhat 
with my hand, the word was diftin&ly feen in the fpedlrum 
in yellow letters on a blue field; and then, on opening my 
eyes on a yellowifh wall at twenty feet diftance, the magnified 
name of BANKS appeared written on the wall in golden cha- 
racters. 
CONCLUSION. 
It was obferved by the learned ML Sauvages (NofoL me- 
thod. Cl. VIII. Ord, 1.)' that the pulfations of the optic artery 
might be perceived by looking attentively on a white wall well 
illuminated. A kind of net- work, darker than the other parts 
of the wall, appears and vanifhes alternately with every pulfa- 
tion. This change of the colour of the wall he well afcribes 
to the compreflion of the retina by the diaflole of the artery. 
The various colours produced in the eye by the preffure of the 
finger, or by a ftroke on it, as mentioned by Sir Isaac New- 
ton, feem like wife to originate from the unequal preffure on 
various parts of the retina. Now as Sir I aac Newton has 
(hewn, that all the different colours are reflected or tranfmitted 
by the laminae of foap bubbles, or of air, according to their 
different thicknefs or thinnefs, is it not probable, that the 
Z z 2 effect 
