88 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



[vol. 45 



ton and for different times of the year and day. It will be seen that 

 in economy of mirror surface the coelostat thus arranged has the ad- 

 vantage. In this form of coelostat the moving mirror is never used 

 in very different positions, so that owing to the consequent probable 

 constancy of figure in the mirror it seems to be well suited to long 

 exposures in stellar photography. 



Comparative Sizes of Mirrors Required to Furnish a 20-inch Horizontal North and 

 South Beam fro7n the Sun at Washington. {^Latitude j8° S3'-) 



It will be noted that at noon of the equinoxes the second mirror if 

 exactly south of the first would cut off the beam, and that at sum- 

 mer solstice it must be further south than in winter to reflect the 

 beam clear over the first mirror. Accordingly the second mirror is 

 provided with a carriage and two pairs of tracks at right angles like 

 the slide rest of a lathe, so that the mirror may be displaced to the 

 west a little before noon when the sun is in its southern declina- 

 tions, and can be shifted back to the east a little after noon.^ The 

 north and south track is for the purpose of shifting the second mir- 

 ror for different declinations of the sun, the mirror being at the 

 south end of the track at the summer solstice. 



As it has been thought that this solution of the difficulties attend- 

 ing the use of the coelostat will prove of interest and value to astron- 

 omers, a large instrument of this type was ordered from the J. A. 

 Brashear Company for exhibition at the Louisiana Purchase Expo- 

 sition to be held next year at St. Louis. Plate xxiii is from a photo- 

 graph of this coelostat as now being tested at the Astrophysical Ob- 



^ With a small instrument of this type it might be more desirable to move 

 the first mirror and driving mechanism on an east and west track for this 

 purpose, but this is not provided for in the large coelostat about to be 

 described. 



