75 



the cameras were mounted on the same polar axis. They were mounted 

 in pairs, each pair covering in duplicate six and one-half degrees, so 

 that the three pairs covered in duplicate a region along the sun's equator 

 twenty degrees long and six degrees wide. By a series of experiments 

 we had found that a plate exposed in one of these cameras for three 

 minutes and forty-five seconds, at a time when the sky was as dark as 



Fig. III. The coelostat and nine-inch lense of the sixty-foot camera. 



it was estimated it would be at the time of totality though fogged some- 

 what by the skylight would show more and fainter stars than if exposed 

 for a shorter time. We had made exposures varying from one to four 

 minutes in the vicinity of Regulus when it was near the meridian begin- 

 ning when Polaris was just visible to the unaided eye. We decided to 

 expose the plates for the intra-mercurial planet for thr^e minutes and 

 twenty seconds. 



