Periscope

The chief objection to an increase in the number of lenses in an instrument is the reduction in the brightness and clearness of the image it involves. The brightness suffers because the amount of light transmitted through the instrument is decreased owing to reflection, absorption and scattering; the clearness is dimin ished because, of the transmitted light, a decreasing proportion is concerned with the formation of the real image, the rest form ing spurious images or appearing as mere fog. When these addi tional lenses belong -to a narrow part of the instrument, their powers, being inversely proportional to the interval between successive lenses—that is, proportional to the inverse square of the diameter of the tube—are high, and their effects in introduc ing irremovable aberrations, such as chromatic aberration and curvature of field, are correspondingly great. From the optical point of view that narrowness in the tube, even for a short length, is very detrimental to the production of a good instrument.

The optical system in a periscope is intended to give a view roughly similar to that obtained by direct vision. Usually angular magnification of one and a half is preferred. An angular field of about 40° is ordinarily attained, with an emergent beam of diam eter 4 or 5 millimeters.

The use of a simple telescope in addition to the top and bottom reflectors would not allow such a field of view to be attained in a periscope of normal length. The typical method of developing more complex telescopes to meet the conditions consists in adding one telescope to another, with their objectives facing one another. The resulting magnifying power is the ratio of the magnifying powers of the component telescopes, and may therefore be kept down to the low value desired, while the leading lenses, which constitute the eyepiece of the first telescope, are of the small diameter needed. A system of this kind will yield satisfactory images near the axis whatever the separation of the two objectives may be ; but in the outer parts of the field, owing to the obliquity of the corresponding beams between these lenses, the brightness of the image is reduced as the separation is increased. There is thus a limit to the length attainable by the use of two telescopes, but the addition of more telescopes gives correspondingly greater lengths.

In

many periscopes a higher power than one and a half, nearly always six, can be introduced instantly in place of the lower power without any change in the focus or displacement of the centre of the field of view. In order to avoid loss of light or reduction of the field as seen by the observer (the "apparent" field), the change of power must be effected at the top of the instrument, that is, in the narrow portion of the tube. This requirement of

course adds to the difficulties inseparable from a narrow tube. Of the many different means of changing the power employed by different makers only one will be illustrated.

When it is required to search the complete sky, the top re flector must consist of a prism, or alternatively of two prisms placed with their reflecting bases close together and parallel to one another. The way in which light from different altitudes is directed down the tube is evident from fig. i.

Principles of Construction.

The accompanying series of illustrations (figs. 2 to 4) show the general principles of construc tion, together with the appearance of the two ends, in one of many patterns made by Messrs. Barr and Stroud. Fig. 2 shows dia grammatically the arrangement of the optical parts in a bifocal periscope giving a field of view of 40c" with a power of i i and a field of r o° with a power of 6. The system will be considered first as a high power instrument. Light enters through the plane parallel plate a, which is sealed into the tube, making a watertight joint ; immediately afterwards it is reflected downwards by the prism b, and is then refracted in turn by the lenses e, f and g, which constitute an inverted astronomical telescope. Of these e corresponds to the eye lens, f to the field lens of the eyepiece and g to the object glass. This telescope is followed by a second, com prising four lenses Ii, k, m and n: It is the objective, k a collector lens, which may be moved along the axis of the tube for focusing and the remaining pair m and n together constitute the eyepiece of the periscope. The lower reflector is the prism 1, to which the lens m of the eyepiece is cemented.

The first telescope produces an inverted image of angular mag nification measured by the ratio of the focal lengths of e and g.

The second telescope magnifies this image in a ratio depending on the relation of the focal length of the pair of lenses It and k to that of the eyepiece m and n. By a proper selection of these focal lengths a power of six can be obtained. The inverted image formed by the first telescope is reinverted by the second, so that objects viewed through the periscope present their natural appearance. To obtain a power of one and a half with a correspondingly augmented field, Messrs. Barr and Stroud introduce a Galilean telescope which gives an upright image in front of the system just described.