14 



NATURE 



\_May I, 1879 



The thread which receives the objectives is of the 

 gauge commonly used in this country, but an adapter can 

 also be supplied which will carry the objectives of 

 Hartnack and other continental opticians. 



The stage is circular and capable of rotation, and it is 

 divided on the margin to 360°. A vernier is attached to 

 the front of the stage, giring readings to one minute. 

 The edge of the stage is milled, and rotation is imparted 

 by hand. 



To insure concentric rotation with any powers used, 

 two screws, carrying milled heads, are connected with the 

 back of the stage. By the employment of these adjusting 

 screws and the cobwebs in the eye-piece, a small object 

 may readily be centred, so that it will revolve about a 

 point central to the field afforded by any objective. 



The object is held either by sliding clamps or by 

 spring clips, and is moved about by hand. With a 

 little practice this simple method of moving the object 

 will be found to answer every purpose. 



The polariser slides into a fitting which is fi.xed to an 

 arm pivotted on the lower, movable surface of the stage, 

 so that it can readily be displaced when ordinary trans- 

 mitted illumination is required, and replaced with equal 

 facility. 



Two little lenses, affording a strongly-convergent pencil 

 of light, are set in metal rings which drop into the top of 

 the fitting which surrounds the polarising prism. When 

 these are employed and the 'analyser is used, without 

 lenses in the eye-piece (a separate fitting is supplied for 

 this purpose), examinations of the rings and brushes, 

 presented by sections of certain crystals, can be advan- 

 tageously carried on, and a quarter-undulation plate can 

 also be employed when needful. The lower end of the 

 fitting which carries the polariser is surrounded by a 

 divided disk, turning beneath a fixed index, so that any 

 position of the prism can be recorded and the rotation 

 imparted to it can be measured. Several other useful 

 pieces of apparatus can be added to the stand at a 

 moderate cost. 



From the foregoing description it will be seen that this 

 instrument is capable of performing the functions of an 

 ordinary microscope, a polariscope, a stauroscope, and, 

 to some extent, a goniometer. A spectroscope could be 

 fitted to it if needful, as well as an apparatus for heating 

 sections of crystals. For a few pounds separate binocular 

 tubes can be supplied, to replace, in a few seconds, the 

 single, but more generally useful, tube. The objectives 

 of any maker can be used with the instrument. 



Having carefully tested one of these microscopes I can 

 speak most favourably of its performance. It is strongly 

 constructed, convenient to handle, and the adjustments 

 work very smoothly. The price of this stand is also 

 remarkably moderate when compared with that of many 

 microscope-stands of far less universal application. It 

 appears to me well qualified to answer the requirements of 

 students of mineralogy and petrology, and it is also ap- 

 plicable to other studies for which microscopes are com- 

 monly required. 



Mr. Watson has taken especial pains to turn out a 

 sound and serviceable instrument, and, after long expe- 

 rience of microscopes, I can confidently say that I have 

 never seen one better suited for the work for which it is 

 designed. Frank Rutley 



STELLAR MAGNITUDES 

 A Request to Astronomers 



THE scales adopted by different observers in their 

 estimates of stellar magnitudes differ considerably 

 from each other, as is well known. As regards the 

 brighter stars, these differences, indeed, are compara- 

 tively unimportant ; but they become larger and more 

 perplexing when the objects observed are faint. Varia- 

 tions of three or four magnitudes may be expected between 



the estimates made of the brightness of minute com- 

 panions seen near a brilliant star. It is needless to point 

 out the inconvenience of this state of affairs, which at 

 times nearly deprives the estimated magnitudes, found in 

 catalogues, of their meaning, and consequently of their 

 value. 



In the hope of providing a partial remedy for this de- 

 fect, a series of photometric observations of stars of 

 various magnitudes, situated near the north pole, has been 

 undertaken at the Harvard College Observatory. The 

 region has been selected as one which may always be 

 conveniently observed in the northern hemisphere, so 

 that the brightness of a star observed in another part of 

 the sky can readily be compared by estimate with any 

 standard polar stars, the relative brightness of which may 

 have been determined by photometric measurements. 



The table and chart given below are designed to serve 

 as guides in finding the stars which are, as has been said, 

 in course of phonometric measurement at the Harvard 

 College Observatory. The stars given in the table are 

 arranged approximately in the order of their brightness, 

 the first being u Ursas Minoris, which is taken in all cases 

 as the standard of comparison, and the next three, 8 Ursse 

 Minoris, 51 Cephei, and X Ursae Minoris. The chart is a 

 copy of a sketch showing the approximate relative position 

 of ten faint stars very near the pole, which are denoted 

 by the italic letters a, b, c, d, e,f, g, h, k, I. The places 



DM. 



89 



89 26 



» 1880. 



4 



of the pole for 1855, '880, and 1900, and of five stars 

 from the Durchmusterung, four of which occur in the 

 table, are also indicated upon the chart, to facilitate the 

 identification of the faint stars. The objects called c and 

 e are nearly in the prolongation of the line through DM. 

 89° 37' and 6. Between these last, and more nearly in 

 the same line than it appears to be in the chart, lies the 

 star a. 



The value and interest of the photometric results to be 

 obtained at the Harvard College Observatory may be 

 greatly increased by the co-operation of astronomers else- 

 where. All who are desirous of improving the present 



