TELESCOPE. 106 TELESCOPE. that the greater the focal length of the lens and the greater its surface, the more satisfactory would be the image produced. This would be the case were it uot for the fact that both spherical and chromatic aberration (q.v. ) increase with the aperture of a lens, and that the larger the piece of glass, the greater the difficulty of secur- ing liomogeneity and freedom from imperfections. Increasing the focal length increases the diffi- culties involved in mounting the telescope as well as in the manufacture of the objective itself. To observe the image formed by the objective an eyepiece is used, whose action is simply to mag- nify the image. The image furnished by the object glass is in- verted and is merely magnified by the eyepiece. In what is known as the terrestrial telescope there is an additional lense or lenses added to erect or invert the image so that it will appear to the eye in its natviral position. This lens merely forms a new image which is then viewed by the eyepiece as in the case of the astronomical tele- scope, the action being shown in the accompany- ing diagram, the erecting lens and the eyepiece being in reality a compound microscope to view the image furnished by the objective. In the diagram C is the object glass, B is the inverted image formed by the rays coming from a distant object in the direction A, E and G serve merely to invert the image B and form it anew and erect at H where it is observed by the eyepiece D. scope, while we have the erecting lens which in- verts the image to its proper position, yet the image is not as bright as in the case of the opera glass, where there is far less magnifying power. The difficulties due to spherical aberration were early experienced by opticians and astron- omers, and in an attempt to obviate them astronomical telescopes were constructed of considerable focal length and power. From ob- servations made with such instruments by Huy- gens, who was the pioneer in this line, he was able to present the first explanation of Saturn's rings (1659). He constructed a telescope 300 feet in length which magnified 600 times, while the telescope used by Cassini to discover the fifth satellite of Saturn (Rhea) was built by Campani of Rome and magnified about 150 times. The diameter of Venus was determined in 1722 by Bradley with a telescope of 212 feet focal length. These telescopes of extreme length were known as aerial telescopes, and naturally their mounting and manipulation presented many diffi- culties. In spite of these awkward conditions, valuable observations were made and ingenious appliances introduced to facilitate the operations. The invention of the achromatic object glass by Dollond in 1757-58 and the improvement of optical flint glass, which commenced in 1754, soon made possible the construction of improved telescopes, but these were all of modest dimen- sions, and until well into the nineteenth E G DIAGRAM BHOWINQ TEEREBTBLAL TELESCOPE. Other arrangements of the terrestrial eyepiece could be mentioned, but the one described is one of the earliest and simplest. ^^■hile the greatest care must be expended on calculating and constructing the object glass, the eyepiece is by no means unimportant. As the magnifying power of the telescope (the ratio of the angles formed by lines drawn to the extremities of the image and the object) is equal to the quotient obtained by dividing the focal length of the objective by that of the ocular, it would be of advantage to use a lens of comparatively large curvature, but here again chromatic and spherical aberration must be considered, for sharpness and distinctness are as essential as high magnification. In prac- tice eyepieces vary considerably, depending on the use to which they are to be put. Generally they consist of two achromatic lenses, one convex, known as the field lens, which brings together the outer rays of the beam, while the lens nearest to the eye may be either positive (convex) or negative (concave). In case a micrometer is used to measure the image a positive lens is employed and the combination, which is known as a Rams- den eyepiece, though not quite achromatic, has a flat field. For mere observation the Huygenian eyepiece, with a negative lens of one-third of the focal length of the field lens, is preferable. The addition of lenses to the eyepiece acts to cut off the supply of light and to destroy the brightness of the image. Accordingly in the terrestrial tele- century few if any object glasses were constructed greater than twelve inches in diameter. The discovery of methods of mak- ing large disks of flint glass was made by Guinand, a Swiss mechanic, who then became associated with Fraunhofer (q.v.), and telescopes as large as 10 inches aperture were readily made. His successors made instruments with object glasses 15 inches across. The next suc- cessful manufacturer of telescope lenses was Al- van Clark of Cambridgeport, Mass., who from the time when an object glass manufactured in his shop was purchased by the Rev. W. R. Dawes of England, gradually achieved the highest rank as a maker of telescope lenses. At the Cambridgeport works have been constructed the lenses not only for the leading American ob- servatories, but also for the Imperial Russian Observatory at Pulkova, and other European institutions. The manufacture of telescope lenses is to be regarded as a fine art, and the ability to work the disks of glass with the requi- site precision and delicacy is possessed by but few. The formation of an image by a concave mir- ror has been made use of in the reflecting tele- scope, of which numerous varieties have been devised and with which many of the most important astronomical discoveries have been made. The principle of this instrument will be apparent from the following diagram and ex- planation:
