X907.] PERTAINING TO WORK WITH A PORTRAIT LENS. 421 



as Rho Ophiuchi. If one examines the space about this star with a 

 telescope he sees nothing remarkable except that there are fewer 

 small stars in this region — yet photography shows us that the sky 

 here is covered by an enormous and magnificent nebula which ap- 

 parently lies in a hole in the sky. From this great vacant region — 

 vacant in the sense of there being few or no stars in it — narrow 

 dark lanes run eastward for many degrees. But the singular thing 

 is that the nebula seems in some way to be responsible for the ab- 

 sence of stars at this point. Whether this is due to the obscuration 

 of the light of the small stars that ought to be here, by the nebula, 

 which would in that case prove it to be nearer to us than the stars, 

 or whether the presence of the nebula has in some way destroyed 

 or dispersed the stars cannot be told. 



Perhaps the most extraordinary revelations of photography in 

 astronomy have been in the case of comets. These wonderful ob- 

 jects with their vast trains sweeping through space are singularly 

 subject to disturbances by other celestial bodies. The photographic 

 plate has shown us that the comets utterly transform themselves in a 

 few hours' time, for, though of vast dimensions, they are in reality 

 but flimsy affairs with little or no solidity. In these changes, so 

 faithfully recorded by photography, they sometimes, through the 

 distortions of their trains, reveal the presence of some kind of re- 

 sisting medium or of some unknown bodies through whose attrac- 

 tion, or by collision with which their tails are twisted, broken or 

 deformed in the most extraordinary manner. This was the case 

 with one of the comets of 1893 where photographs on successive 

 nights show the tail disrupted and broken, undoubtedly by such an 

 encounter. What this really means we have yet to learn. Possibly 

 the comet passed through a dense swarm of meteoric bodies in its 

 flight around the sun. Photographs of another comet showed the 

 tail entirely separated from the head and drifting away in space. 

 From these last pictures it was shown that the particles forming the 

 tail were leaving the comet with a velocity of twenty-nine miles a 

 second. 



Importance of the Portrait Lens. 



The strangest thing in connection with these statements is that 

 the greater portion of these photographic revelations have been made 



