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SCIENCE 



[N. S. Vol. XXXVI. No. 927 



ing for the Keeler reflector, and attached 

 to the telescope is a powerful solar spec- 

 trograph, the gift of Mr. H. K. Porter. 

 With these we are taking part in the spec- 

 troscopic determination of the solar rota- 

 tion, a project that was set on foot at the 

 last meeting of the International Solar 

 Union, and in which six observatories, in 

 this country and abroad, are cooperating. 



Under the southeast dome is the thirteen- 

 inch visual refractor that formed the nu- 

 cleus around which the old observatory 

 was built. Once the third largest telescope 

 in the world, it has now become the third 

 largest in our observatory. This telescope 

 we now use chiefly for the instruction of 

 the public; with the lecture hall below it 

 (used on cloudy evenings) it forms a pub- 

 lic observatory, the privileges of which are 

 freely offered to the people of Pittsburgh. 

 So eagerly has this offer been accepted, 

 that two years ago we found it expedient 

 and possible to extend this work, and to 

 pledge ourselves to continue it in per- 

 petuity. 



Attached to the thirteen-inch refractor is 

 a four-inch camera, used to determine the 

 brightness of stars by the extra-focal 

 method. The observing program is made 

 up chiefly of stars that we are observing 

 at the same time with the spectrograph. 



In one of the basement rooms on the 

 north side of the building a ten-inch pho- 

 tographic telescope is mounted in a fixed 

 position on an inclined pier and directed 

 toward the north pole of the heavens. The 

 work with this instrument is of an experi- 

 mental character. If its outcome should 

 be favorable we hope to undertake, prob- 

 ably in cooperation with the Harvard Col- 

 lege Observatory, the compilation of a 

 catalogue of faint stars by entirely new 

 methods. 



At the west end of our building is a 

 four-inch transit instrument with which 



we are still maintaining the extensive time- 

 service installed by Langley in 1869. As 

 auxiliaries to this instrument we have 

 three second-pendulum clocks. One of 

 these is a Eiefler clock maintained under 

 constant pressure and temperature, and 

 this proves to be a time-piece of unusually 

 accurate performance. 



Lastly we come to the Thaw memorial 

 telescope, under whose dome we are as- 

 sembled this afternoon. Last summer 

 when we fixed the date for this dedication, 

 we thought that this telescope would be 

 quite complete by to-day; but it appears 

 that at least another year must elapse be- 

 fore the objective can be ready. This is 

 due to the difficulty of securing a suitable 

 disk of flint glass, the crown disk having 

 been delivered some months ago. The 

 aperture of the telescope is to be not less 

 than thirty inches, and unlike most other 

 refractors of the largest size, it is to be 

 primarily a photographic instrument. A 

 twelve-inch correcting lens is to be pro- 

 vided; it will be a matter of only a few 

 seconds to put it into the optical axis, thus 

 changing the color curve into one that will 

 be suitable for visual observations. The 

 mounting has all those appurtenances (and 

 no other) that modern practise has shown 

 to be desirable. Throughout its design and 

 construction, efficiency for astrometric work 

 was the chief object in view. 



With the Thaw telescope we contemplate 

 an attack upon three problems; first, the 

 accurate determination of the distances of 

 many stars. How extensively we shall go 

 into this work will depend upon the activi- 

 ties of certain other observatories that 

 have declared similar intentions. But the 

 need of such determinations is one of the 

 most pressing in astronomy and it is likely 

 that the telescope before us can be profit- 

 ably occupied in this work for many years 

 to come. Secondly, we owe it to our sue- 



