Optics and observations

The concluding paragraph in this letter, from one of the most able men of the age, encouraged me to examine and compare particular instances, as they fell under my own observation, and from a variety of these I am led to conclude, that, among those numerous causes here said to operate, independent of mathematical principles, one may proceed from the position of the eye itself; which is so placed as to view a certain portion of the hemisphere without any motion of the head. This portion has been differently stated by different authors, varying from sixty to ninety degrees.