Few persons can see with the naked eye - much less measure - more than six stars of the Pleiades, although all the stars measured by Maestlin have been seen with the naked eye by a few individuals of exceptional powers of eyesight.
If the origin of light be treated as infinitely small, and be seen in focus, whether with the naked eye or with the aid of a telescope, the whole of the light in the absence of obstacles would be concentrated in the immediate neighbourhood of the focus.
A lenticel appears to the naked eye as a rounded or elongated scar, often forming a distinct prominence on the surface of the organ.
The cercaria is just visible to the naked eye and has an oval or discoidal body and usually a long tail of variable form.
The movements of the pollen-masses may readily be seen with the naked eye by thrusting the point of a needle into the base of the anther, when the disks adhere to the needle as they would do to the antenna of an insect, and may be withdrawn.
In front of the naked eye was held a piece of copper foil perforated by a fine needle hole.
Since this shortsighted observer can view the object with the naked eye with no inconvenience to himself at 4 in.
Distant that the two perspectives formed by the naked eye are no more distinguished from each other, recourse may be had to binocular telescopes and range-finders; and if the objects be so small that, in order to observe details on them, we must bring our eyes so close to the objects that they cannot accommodate the images, recourse may be had to binocular microscopes and magnifying glasses.
The whole French army, and even Napoleon himself with his staff, were not on the far side of the streams and hollows of Sokolnitz and Schlappanitz beyond which we intended to take up our position and begin the action, but were on this side, so close to our own forces that Napoleon with the naked eye could distinguish a mounted man from one on foot.
The faintest stars visible to the naked eye on clear nights are of about the sixth magnitude; exceptionally keen, well-trained eyes and clear moonless nights are necessary for the perception of stars of the seventh magnitude.