TELESCOPES. 



655 



with the Lick trustees for one of 36 in. The glass for 

 all these large lenses is of foreign manufacture, coming 

 from either Chance & Co.. of Birmingham, or Feil & 

 Co., of Paris; and considerable difficulty was expe- 

 rienced in obtaining clicks of the requisite degree of 

 purity, particularly in the case of the 36-in. crown 

 glass disk, which involved nearly three years' labor and 

 19 failures before a suitable piece was obtained. 

 Of smaller objectives the Clarks have made a great 

 number over 60, probably, between the apertures 6 

 and 12 in. 



The mounting for the 30-in. Pulkowa objective was 

 made by the Repsolds, of Hamburg ; that lor the 

 Lick glass by Warner & Swasey, of Cleveland, Ohio. 

 The following details of the Lick mounting are taken 

 in part from an illustrated description of the instru- 

 ment given in Enyuicering for Aug. 17, 1888 : 



The telescope is supported on a hollow cast-iron 

 column of rectangular section, measuring 4 ft. by 8 ft. 

 at the top, 5 ft. by 9 ft. at the floor line ; then spread- 

 ing rapidly to a base 10 ft. by 16 ft. bearing on the 

 foundation. Around the top is a balcony, to which 

 access is given by a spiral staircase. The objective in 

 its cell weighs about J ton, the column and head 

 which carries the polar axis weigh together '21 tons, 

 while the total weight of the telescope complete 

 is 40 tons. The polar axis is of steel, 10 ft. long, 12 

 in. in diameter at its upper and 10 in. in diameter at 

 its lower bearing, and having a hole f> in. in diameter 

 extending its entire length. Through this hole pass 

 the shafts by which the observer at the eye end work.-i 

 the right ascension clamp and slow motion. The de:- 

 lination axis is also of steel, 10 ft. 6 in. long., 10 in. in 

 diameter at its upper and 9} in. at its lower bearing, 

 arid has a 4-in. hole extending through it. Bearings 

 of Babbitt metal are provided for both axes, but much 

 of the weight is carried by anti-friction rolls and an 

 ingenious arrangement of "ball bearings." The tel- 

 escope tube consists of a central section of cast-iron 

 strongly ribbed, to which sections of sheet-steel arc 

 attached. When the telescope is horizontal the flex- 

 ure of the tube with the object glass and permanent 

 counter-balancing weights is J in. only, while with a 

 load of one ton added at each end the flexure is in- 

 creased to J in. 



An observer at the eye end can perform the follow- 

 ing operations : 



1. Clamp in declination. 



2. (live slow motion in declination. 



3. Read the declination circle (2 verniers). 

 .4. Clamp in right ascension. 



5. Give slow motion in right ascension. 



6. Start or stop driving clock. 



7. Read the right ascension circle (1 microscope). 

 An assistant on either side of the balcony can 



8. Clamp in declination. 



9. Give quick motion in declination. 

 JO. Give slow motion in declination. 

 )1. Clamp in right ascension. 



12. Give quick motion in right ascension. 



13. Give slow motion in rijrht ascension. 



14. Stop or start driving clock. 



15. Head right ascension circle (2 microscopes). 

 ]>'>. Head a dial showing approximate declination. 

 There are three permanent finders of 6 in., 4. in., and 



2| in. diameter respectively, and brackets are provided 

 for attaching a 12-in. telescope belonging to the observ- 

 atory when especially accurate pointing, as in some 

 photographic work, is required. The eye end is 

 further provided with a fine position micrometer made 

 by Faulh & Co.. and a stellar spectroscope by Bra- 

 fhear. The driving clock is in the upper section of 

 the pier. The speed of the clock is regulated by a 

 {'fictional governor of the cross-armed type and by an 

 electric control, by means of which its nitc is kept in 

 nent with that of a standard sidereal clock. One 

 of the arbors, turning once in a minute, curries a chro- 

 nograph drum. _ . . 



For use as a photographic telescope a third (crown) 

 lens of 33 in. clear aperture, weighing with the cell 

 150 pounds, is mounted in front of the visual object- 

 ive, its application shortening the focal length of the 

 telescope by 10 ft. An opening is therefore made in 

 the tube 10 ft. from the eye end, and giving access to 

 a pkia-holder capable of carrying a plate 20 in. square, 

 and provided with all the necessary adjustments. An 

 iui age of the moon upon the plate is about 5J in. in 

 diameter. 



The dome covering this great telescope is 70 ft. in 

 diameter, and the floor can be raised vertically through 

 a distance of 16J ft. by four hydraulic rams. The 

 observing slit is yj ft. wide, closed by two steel shut- 

 ters, which are opened by a pull of 5 Ibs. upon an 

 endless rope. The cost of the visual objective was 

 $52,000, that of the photographic corrector $13,000, 

 while the mounting was furnished for $42,000, and 

 the dome and movable floor were put in place for 

 $73.300. 



We have given a somewhat detailed description of 

 this instrument for the reason that it may be regarded 

 ' as embodying on a large scale all of the modern im- 

 j proveuients and conveniences that have been introduced 

 1 into telescope construction. Both Warner & Swasey 

 and Fauth & Co. have furnished mountings for a num- 

 ber of Clark's smaller lenses, which are all that could 

 I be desired for ease of manipulation and steadiness. 



Among improvements in astronomical instruments 

 should be mentioned the '' photolieliograph," a horizon- 

 tal telescope of long focus, which was first brought into 

 DM by Prof. Joseph Winlock in 18G9-7()at the Harvard 

 College Observatory. A somewhat similar apparatus 

 was previously suggested by Capt. Laussedat and others 

 in France, but the peculiar advantages of this form of 

 instrument for photographic observations of the sun 

 do not. seem to have been appreciated until the results 

 of independent experiments at Cambridge had been 

 published. Photoheliographs >f 5 in. aperture and 

 40 ft. focal length, mounted in the meridian, were used 

 by all the parties sent out by the United States to 

 observe the transits of Venus of 1S74 and 1880, and 

 the superior results obtained by these parties seem to 

 have justified the adoption of the "American method" 

 of observation. A form of horizontal mounting for 

 large reflectors, which appears to possess some of the 

 advantages of Loewy's "equatorial coudeV' was de- 

 scribed by Mr. J. A. Hill at the meeting of the Amer- 

 ican Association for the Advancement of Science in 

 1871. Tin: astronomer is enabled "to remain seated 

 in one position, looking in one direction, and entirely 

 protected from cold, fatigue, night air. etc." Prof. 

 Pickering has used a horizontal telescope mounted at 

 right angles to the meridian in his extensive photo- 

 metric work, and for the continuation of this work to 

 fainter stars he has recently had constructed by the 

 Clarks an instrument of this kind of 12 in. aperture 

 and 2DO in. focal. The object glass is at the western 

 end of the tube, and in front ol' it is placed a plane 

 mirror. 18 in. in diameter, so mounted that the light 

 of a celestial object not more than one hour either side 

 of the meridian can be thrown into the field of the 

 telescope. _ The eastern end of the tube, carrying the 

 eye-piece, is covered by a small building, which can be 

 warmed. An auxiliary telescope of 5-in. aperture is 

 employed to bring into the field an image of the 

 pole-star, which is reduced by polarizing apparatus to 

 equality with the image of any star observed in the 

 principal telescope. The angular apertures of the two 

 telescopes are such that the emergent pencils may be 

 coincident. 



Photography has recently been developed into such 

 a valuable accessory in astronomical observations that 

 it liccomes extremely desirable to provide means by 

 which the ordinary visual objective inay be readily 

 transformed into a photographic objective, for the cor- 

 rection for chromatic- aberration by means of the flint 

 glass lens in the ordinary objective is too great to give 



