SPECTROSCOPE 



5488 



SPECTRUM ANALYSIS 



are diverged and a clear image is formed on 

 the retina. For farsightedness, the convex 

 lens is used, converging the light rays. The 

 thicker the lens the greater is its magnifying 

 power. Astigmatism, which is a structural de- 

 fect of the eye, is remedied by cylindrical 

 lenses which bring the rays of light to a com- 

 mon focus on the retina. The lenses are ad- 

 justed so as to make the distance of distinct 

 vision about twelve inches from the eye. 



When different lenses are needed for examin- 

 ing distant objects and those near the eye, di- 

 vided spectacles are used. In these, each glass 

 consists of two semicircles of different focal 

 power; the lower and smaller section is used for 

 reading and close observation and the upper sec- 

 tion for looking at distant objects. When cor- 

 rectly fitted, spectacles can remedy most defects 

 of vision, but if not exactly suited to the eye, 

 they are injurious rather than beneficial. 

 Therefore they should be fitted by a skilled op-, 

 tometrist and be changed when the eye 

 changes. Colored glass is used in spectacles to 

 .protect the eyes from the glare of the sun. 



The frame is made of tortoise shell or metal, 

 usually gold, silver or steel. It consists of a 

 bridge, rims, sides and bows which fit over the 

 ears. Lenses supported on the nose by means 

 of a spring and having no sides and bows are 

 called eyeglasses. See LENS; EYE. C.B.B. 



.Consult Pettet's Mechanics of Fitting Glasses; 

 Prentice's Ophthalmic Lenses. 



SPECTROSCOPE, spek ' tro skohp, an instru- 

 ment for studying the spectra of bodies to de- 

 termine their composition. (For a full ex- 

 planation of the term spectrum see subhead 

 under LIGHT, page 3425.) The simplest form 

 of spectroscope is 

 a triangular glass 

 prism, and the 

 method of using 

 it is shown in the 

 diagram under 

 SPECTRUM ANALY- 

 SIS (which see). 

 In a spectroscope SPECTROSCOPE 



of this sort the Explanation of the figure 

 colors overlap. a PP ears in the article. 

 For the purpose of analysis it is necessary that 

 each color be sharply defined, and this defini- 

 tion is secured by the use of a telescope and 

 other tubes in connection with the prism. 



The spectroscope in general use is shown in 

 the accompanying diagram. It consists of a 

 prism (p) enclosed in a case to which three 

 tubes are attached. A tube (c), called the 



collimator, has a convex lens at the end nearest 

 the prism and a slit at the other end. This 

 slit is at the focus of the lens, so that the light 

 admitted through it enters the prism in parallel 

 rays. The slit is formed by two pieces of 

 metal, one of which is movable and is operated 

 by a screw with a very fine thread; thus the 

 amount of light admitted to the prism may be 

 carefully regulated. The colored image of the 

 slit is magnified about eight times and is 

 viewed through the telescope (b), which gives 

 a sharply defined image. The tube d con- 

 tains a graduated scale, the image of which is 

 thrown upon the spectrum. 



The compound spectroscope used in obtain- 

 ing spectra of the heavenly bodies consists of 

 a series of prisms arranged in the arc of a circle 

 so that the spectrum is magnified by each be- 

 fore it is viewed by the observer. These spec- 

 troscopes are so constructed that they can be 

 attached to the eyepiece of a large telescope. 

 Astronomers measure distances and velocities 

 of stars by means of the spectroscope. The 

 dark lines appearing in the spectra of glowing 

 bodies move towards one or the other end of 

 the spectrum according as the body under in- 

 vestigation is approaching the observer or re- 

 ceding from him. Upon this principle astrono- 

 mers have based some of their most remarkable 

 discoveries. G.L. 



Consult Newall's The Spectroscope and Its 

 Work. 



SPECTRUM, spek'trum. See LIGHT, sub- 

 head The Spectrum. 



SPECTRUM ANALYSIS. The effect of 

 passing a beam of sunlight through a glass 

 prism in a dark room is shown in the diagram on 

 page 5489. The image of rainbow colors formed 

 by this experiment is called the solar spectrum 

 because it is produced most perfectly by sun- 

 light, but it has been found that any glowing 

 substance, such as molten iron or burning hy- 

 drogen, will produce a spectrum. Moreover, each 

 substance forms a spectrum different from that 

 of any other substance; therefore, by study- 

 ing the spectrum of any substance we can learn 

 its chemical composition. The term spectrum 

 analysis is applied to this sort of study. 



A spectrum that shows an unbroken array of 

 colors from red to violet is a continuous spec- 

 trum, like that formed by sunlight. The spec- 

 trum formed from a single substance, like hy- 

 drogen, for instance, or that from a number of 

 gases, is not continuous, but shows bands or 

 lines of color separated by dark spaces, and it 

 is known as a bright-line spectrum. Experi- 



