154 STELLAR EVOLUTION. 



optician decides to produce an iniai^e nio.st suitable for eye observations 

 he delil)erately discards the blue and violet rays, simph' because tliej" 

 are less ini])()rtant to the eye than the yellow and green rays. For 

 this reason the iniag-e of a star produced by a large refracting telescope 

 is surrounded ])y a 1)1 ue halo containing the rays discarded l>v the 

 optician. These very rays, however, are the ones to which the ordi- 

 nar}^ photographic plate is most sensitive; hence in a photographic 

 telescope the blue and violet rays are united, while the yellow and 

 green rays are discarded. 



The 40-inch telescope is of the lirst type, constructed primarily for 

 visual observations. In order to adapt it for photography Mr. G. W. 

 Ritchev, of the o})servator3^ staff, simply places before the (isochro- 

 matic) plate a thin screen of yellow glass, which cuts out the blue rays, 

 but allows the yellow and green rays to pass. As isochromatic plates 

 are sensitive to yellow and green light, there is noditficulty in securing 

 an image with the rays which the oljject glass unites into a perfect 

 image. During the entire time of the exposure a star which lies just 

 outside the region to be photogiuphed is observed through an eye- 

 piece magnifying l.OUO diameters. This eyepiece is attached to the 

 frame which carries the photographic plate, and is sasceptibk; of 

 motion in two directions at right angles to each other. In the center 

 of the eyepiece are two very iine cross hairs of spider wel) illuminated 

 by a small incandescent lamp. If the observer notices that through 

 some slight irregularity in the motion of the telescope, or through 

 some change of refraction in the earth's atmosphere, the star image 

 is moving away from the point of intersection of the cross hairs, 

 he instantly brings it back b}' means of one or both of the screws. As 

 the plate moves with the eyepiece it is evident that this method fur- 

 nishes a meiuis of keeping the star images exactly at the same point 

 on the plate throughout the entire exposure. With such apparatus 

 certain data are gathered for the study of stellar development. 



It is easier to trace the successive steps in the development of a star 

 after it has been formed than it is to account for its origin, but all the 

 evidence that has ))een accunuUated up to the present time tends to 

 show that stars are condensed out of the cloud-like masses whi<'h we 

 know as nel)ulii'. Less than half a century has passed since the true 

 nature of a gaseous n('l)ida was determined. In his extensive obser- 

 vations of astronomical plieiioinena Sir William Herschel examined a 

 great number of star clusters similar to that shown in Plate 1\'. His 

 telescope was a large one, but it can safely be said that he nm cr saw a 

 cluster so well as this object can be perceived through the aid of pho- 

 tography, lie found in studying object aftei- o])ject in all parts of the 

 heavens that many (lusters could be ivsolved into their constituent 

 stars. In some of lliesc clusters the stars are widely sejiaraled by a 

 powerful instrument, as (hey :i])pear in this photograph. In others, 



