There is no theoretical limit to the magnifi cation attainable with simple microscopes ex cept that set by the nature of light itself, but very high powers demand inconveniently small lenses and such close approximations to the eye that illumination of the object becomes difficult. These difficulties can be greatly re duced by employing two systems of positive lenses, one of which serves to form a real and enlarged image of the object, while the other is a simple microscope used to observe the real image quite as if it were the object itself. The former system is called the objective, or object-glass, and the latter the ocular, or eyepiece. The instrument thus constituted is called the compound microscope, and its magni fying power is equal to the product of that of the objective into that of the ocular. The com pound microscope was invented about the mid dle of the 17th century, but it was not perfected so as to be of real value as an instrument for scientific research until after the second dec ade of the 19th.
If a lens, or lens system, is employed to i form a real image of a distant object and this image be viewed through a microscope, the combination forms a telescope. Since the real image is in general inverted, the object ap pears inverted to the observer if he employs a simple magnifier as an ocular, but it appears erect if the ocular is a compound microscope. The inverting telescope is optically superior and is universally used for astronomical obser vations, but for terrestrial observations the second type is ordinarily preferred, which,. when it is desired to distinguish it by a name, is called a terrestrial telescope, or a spy-glass. The terms objective and ocular are also applied to the two lens systems in the telescope.
Since concave mirrors can also produce real images of distant objects they may be used in place of the objective. Such instruments are called reflecting telescopes; they have been very extensively used for astronomical purposes in the past.
Achromatism — Achromatic Combina Newton was led by his experiments to conclude that the secondary phenomenon of dispersion bears a constant ratio to the refrac tion. It follows from this that a separation of composite light into colors• is the inevitable concomitant of change of direction by refrac tion, and that this imposes a somewhat narrow limit upon the power of all optical instruments involving refraction. This belief led him to invent the reflecting telescope, which remained the leading form for astronomical purposes for more than a century. About the middle of the 18th century, however, Dolland found that Newton's conclusion was founded upon too limited a range of experiment and showed that it is possible, by a combination of two or more materials, to secure a change of direction of light by refraction with little or no evident dispersion. Thus, he found that a prism of crown glass, say of 10 degrees, combined with a prism of flint glass of 5 degrees, turned in an opposite direction, would yield a deviation nearly as great as a prism of crown of 5 de grees and without the colors of dispersion. Such a combination is called an achromatic combination, and a pair of lenses similarly combined to give colorless images is called an achromatic lens. All refined optical instru
ments utilize this invention of Dolland. In telescopes the objective ordinarily consists of two members only, a positive crown lens com bined with a negative (diverging) flint lens; in microscopic objectives there are rarely less than four lenses, and sometimes, in the case of very high powers, not less than 10.
Interference Phenomena — Diffraction.— That light is in fact some form of wave-motion does not appear from the phenomena of reflec tion and refraction in their commoner mani festations, although if the acting surfaces are made very small there are deviations from the simple laws given above which inevitably lead to a wave theory for an adequate explanation. For the present purposes it seems far better to describe some of the simpler and easily produced phenomena which demonstrate the wave-motion.
One of the most striking properties of all varieties of waves is theirpropagation independ ently of the state of motion of the medium in which they exist, for example, water waves of all possible lengths and having all possible di rections of propagation may coexist on a sin gle surface of water. In familiar acoustic phe nomena we have excellent analogies which will greatly help in the comprehending of the less familiar optical phenomena. If two tuning forks of exactly the same pitch be sounded to gether it is found that there are regions where all evidence of sound vanishes, provided that the forks are equally loud. These regions of silence are those where the maximum of density due to one set of waves corresponds with the minimum of the other set, and they are so sim ply related to the positions of the two forks that having established their places, it is easy to deduce from the geometrical relations the length of the waves. Could we get two sources of light which emitted waves exactly alike, there should be corresponding regions where illumination from the two sources would be wanting, provided that light is in fact pro duced by a wave-motion. Since, however, the ultimate sources of light are the molecules of luminous bodies and we are unable to control such small bodies, so simple a test is impos sible; but a perfect optical image of a source is exactly like the source itself, hence, if light is allowed to fall upon a screen and a portion of the same radiation is reflected by a mirror upon the screen, the conditions for interference are met. The experiment is a delicate one and liable to escape observation, only, however, be cause of the shortness of the waves. Fresnel tried the experiment, not only by using two mirrors, enclosing an angle a very little less than 180 degrees, but also by using two prisms of very small angle, with complete success. The advantage of using a pair is obvious when similarity of the two sources — here two virtual images of the same source — is considered. His measurements showed that the waves which are found in ordinary white light have all lengths between those of • s loo and nine of an inch, the former being that of the extreme red of the prismatic spectrum, and the latter of the extreme violet. The mean value for white light may be taken as of an inch.