Studying the Stars with Mirrors 



The biggest reflecting telescope 

 in the world belongs to Canada 



By Dr. C Furness 

 Professor of Astronomy in Vassar College 



IT IS witli the 

 reflecting tele- 

 scope that many 

 of the most brilliant 

 discoveries about 

 stars are made. Us 

 construction, how- 

 ever, is not so gen- 

 erally understood as 

 that of the refract- 

 ing telescope, the 

 form of instrument 

 which is so often 

 seen in the parks or 

 on the streets of our 

 cities and through 

 which the passerby 

 can get a peep at 

 the Moon for the 

 trilling sum of five 

 or ten cents. By 

 calling attention 

 first to certain facts 

 regarding this more 

 familiar type of 

 telescope, it will be 



easier to make clear the construction of the 

 reflecting telescope. 



The lens at the upper end of a refracting 

 telescope is called the object glass. It 

 collects the rays of light and brings them 

 together at a focus to form an image, 

 which is viewed with a magnifNing eye- 

 piece. The largest refracting telescope is 

 the well-known Yerkes instrument. It 

 has an object gla.ss forty inches in diameter. 



In a reflecting telescope, the light is col- 

 lected by reflection from the surface of a 

 concave mirror. If this surface is ground 

 to a parabolic shape, the ra\s will all come 

 together at a single point to form an image, 

 just as with the refracting telescope; but 

 this point will be situated on the same side 

 of the mirror as the object, and hence the 

 observer who tries to look at a star will 

 find his head in his own line of vision. In 

 order to overcoi7ie this dilficully, a second 

 reflection is made to lake place, so as to 



Above : Spiral 

 nebula Messier 101, 

 Ursae Majoris, 

 photographed with 

 the two - foot re - 

 flector of the Yerkes 

 Observatory. Time 

 of exposure, three 

 hours 



Another photo- 

 graph of the same 

 nebula, taken with 

 the sixty-inch re- 

 flector of the Mt. 

 Wilson Solar Ob- 

 servatory. Time of 

 exposure, seven 

 hours, thirty min- 

 utes. Comparison 

 shows the greater 

 detail in the 

 lower photograph 



deflect the beam of light and form the 

 image at one side of the tube, where it may 

 easily be examined with an eyepiece. This 

 second reflection is accomplished b\- means 

 of a ])Iane mirror or "flat" inserted in the 

 upper end of the tube and set at an angle 

 of 45°. This flat will neccssarih- cut oft' 

 some of the light falling u|Xin the (principal 

 mirror, but since it is not large and since 

 its sujjports are made as slender as possible, 

 there is no serious loss. 



A Mirror Six Feet in Dia meter 



At first mirrors were made of speculum 

 metal, an allo\- of copper and tin, which 

 can i)e very highly ])olished. As early as 

 1842, the famous reflector of Lord Rosse 

 was constructed. It had a mirror six feet 

 in diameter. With this instrument many 

 dr.iwings of nebulae and planets were made. 

 Howexer, it never attained tiie usi-fiilness 

 wliirli might have been expected, chielh' on 



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