H 



NA TURE 



\_May 5, i88 



to easily turn and set the instrument to right ascension. 

 This can be understood from Fig. 3, which is from a 

 photograph. In the case of an instrument of the size of 

 the Vienna equatorial, the observer requires an elaborate 

 stage or platform of variable height and position in order 

 to reach the eye-end, and if he has to move to another 

 object he must descend from his chair or platform and 

 move // before he can move the instrument, and then he 

 has not the facility of sighting objects, but must adopt a 

 sort of tentative process, climbing up and down his sta^'e 

 and moving it and the telescope alternately. To avoid 

 this labour and save time an arrangement is supplied by 

 which the assistant from the ground floor can set the in- 

 strument in right ascension, and once set in one direction, 

 the other (declination setting) can be readily managed by 

 the observer. The assistant is supposed to stand at the 

 south end of the pier. He has there before him a desk 

 on which his catalogue and paper can rest, a sidereal 

 clock-face let into the south end of the pier, a reader tele- 

 scope by which he can read the lower right ascension 

 circle, and a hand-wheel, which by means of shafts and 

 gearing communicate motion to the instrument in r\^h.1 

 ascension, and finally a handle which is keyed on to a 

 screw forming the toe-bearing of the quick motion diiv- 

 '"g-shaft. By giving this handle one turn, the driving- 

 shaft IS allowed to drop down out of gear with the rest of 

 the wheelwork, so that the clock when driving the instru- 

 ment may not have the additional work to do of driving 

 this shaft, which is necessarily a quick one as compared 

 to the motion of polar axis itself. A lamp is attached to 

 pier at west side, which serves to throw light on sidereal 

 clock-face, catalogue, &c., on desk, and also to send a 

 beam of light up through a long tube directed to the 

 vernier of the right ascension circle which is visible 

 through the reader. The assistant has therefore full 

 povver of setting the instrument roughly, or if desired 

 with any degree of accuracy, in right ascension, or read- 

 ing off the right ascension of an object which requires to 

 be identified. 



The tube of the telescope is made of steel plate about 

 one-eighth of an inch thick in the centre, and taperino- to 

 about one-twelfth of an inch at the ends, the points being 

 all lapped and riveted ; it is nk feet in length, and les- 

 sens from 36;V inches in diameter in the centre to 27 inches 

 at the one and 12 inches at the other end. The weight of 

 the moving parts is between six and seven tons, and still 

 the whole is under easy control of the muscular power of 

 one arm. 



The Commissioners appointed by the Austro- Hungarian 

 Government to report on this telescope consisted of Prof 

 Bal, Astronomer- Royal for Ireland, Earl of Crawfurd and 

 Balcarres, Mr. Huggins, Prof J. Emerson Reynolds, 

 Ear of Rosse, Prof Stokes, Dr. G. J. Stoney, and Mr. 

 \n \! Austro-Hungarian Consul at Dublin. On 



March 16 this Commission forwarded to the Austro- 

 Hunganan Embassy in London their full approval of the 

 performance of the instrument, thus marking the success- 

 ful completion of the largest refracting telescope in 

 existence. 



It will be remembered that Mr. Grubb has built amono- 

 other fine instruments the great Melbourne Reflector, the 

 largest equatorially-mounted telescope known. He has 

 not rested on his laurel;, but is now to be cordially con- 

 gratulated on a still greater accomplishment. 



WHEWELL ON COLOURING GEOLOGICAL 

 MAPS 

 r^NE of the subjects to be brought forward for dis- 

 ^^ cussion at the International Geological Congress to 

 be held this year at Bologna ii the unification disfigures; 

 that IS to say, an attempt will be made to come to some 

 agreement as to the signs and colours to be used on 

 geological maps. Two things have to be indicated: 



succession, i.e. the stratigraphical position of the rock ; 

 and formation, by which, adopting a terminology now in 

 much favour on the Continent, we mean the lithological 

 character and origin of the rock, and not, as is generally 

 understood by the word in this country, a subdivision of 

 the stratigraphical series. I have found among Prof. 

 Sedgwick's papers a scheme drawn up by Dr. Whewell 

 nearly half a century ago, which I have thought might be 

 of use to those who are considering this question, and of 

 interest to many besides. It is dated from Dublin, 1835, 

 where he was attending the meeting of the British Asso- 

 ciation, and It was probably suggested by the publication 

 of Griffiths' map. 



William Smith shaded up to the lowest beds of the 

 various groups into which he divided the strata, in order 

 by contrast to mark more strongly the coming in of a new 

 series, but he does not seem to have adopted any system 

 beyond sometimes taking such tints as were suggested by 

 the predominant colours of the rocks represented. Salter, 

 I remember, proposed a scale of colours founded on the 

 spectrum, but the scheme proposed by the late Master of 

 Trinity I do not remember to have seen put forward 

 before. thos. McK. Hughes 



Trin. Coll., Cambridge, .'Vpril 26 



Proposal for a Systematic Scale of Geological Colours 



The objects which it would be desirable to secure in 

 fixing the scale of colours for a geological map appear to 

 be the following : — 



1. That the different members of the geological series 

 should be coloured in a manner somehow depending on 

 their place in that series, so that successive rocks are 

 distinguished. 



2. That this distinction (of succession) should be 

 governed by some general principle, and 'should not be 

 merely arbitrary ; so that without referring to the inde.x 

 list the colour itself should show the difference of older 

 and younger in the rock. 



3. That the colours should also show the great leading 

 differences of the material of the rtrata (limestone, sand- 

 stone, clay), so that without referring to the scale these 

 differences should be known from the colour. 



4. That igneous rocks should by some general circum- 

 stance in the colour be distinguished from sedimentary. 



5. That the colours which are brought near each other 

 by proximity of succession should be strongly contrasted. 



The following method would, I think, secure these 

 objects. It proceeds on the supposition that there are 

 three primary colours — Red, Yellow, Blue — and that any 

 two of these mixed in considerably different proportions 

 make several shades of intermediate colour; thus between 

 red and yellow we have many shades of orange, proceed- 

 ing from pure red, through reddish orange, to orange, 

 yellowish orange, and pure yellow; in like manner we 

 have many shades of green between yellow and blue, and 

 many shades of purple between red and blue. Our scale 

 can be subdivided as far as these shades are distinguish- 

 able. We have also black, which can be combined in 

 various proportions with each of the simple colours ; thus 

 black with successive doses of red makes brown more 

 and more red. 



The general principles which I propose are these : — 



1. Let one of the above four simple colours represent 

 the material and another the order of succession, and let 

 successive mi.xtures represent successive strata of the 

 same material. Thus let the oldest limestone be pure 

 blue, let doses of yellow be added to mark newer and 

 newer limestones, the chalk being a very >ellow green ; 

 thus all the oolite series will be green of different shades. 



2. Let the simple colour which represents succession 

 be different for different materials, thus let blue represent 

 succession for the clays, and let all the clays be purple, 

 the oldest therefore being the reddest, and the newest the 

 bluest purple. 



