520 



NATURE 



[March 31, 1898 



Concave Gratings for Stellar Photography. — Some 

 experiments have been carried on quite recently at the Johns 

 Hopkins University to investigate the value of the use of con- 

 cave gratings for stellar spectroscopy, and the results obtained 

 bid fair for further trials {Astrophysical Journal, vol. vii. No. 3, 

 March). The methods originally suggested by Prof. Rowland 

 have been developed ; Dr. Poor has derived the formulas, and 

 directed the construction of the apparatus, while Mr. Alfred 

 Mitchell has made the experiments and photographs. The 

 method finally adopted was the direct one, the grating being the 

 objective and spectroscope combined ; the light from the star 

 was thus reflected directly from the grating to the photographic 

 plate. The best position for general work was found to be that 

 in which the centre of the photographic plate falls on the axis of 

 the grating. From the simplified general equation 



I + cos V 



in which p is the radius of curvature of the grating, R and v the 

 spherical coordinates of the light source, and r and ft. those of the 

 curve on which the spectra are brought to a focus (R being oc 

 and IX. = o), it was found that those parts of the spectra where 

 cos iJL could be assumed equal to unity, were brought to a focus 

 on a circle whose radius is given by the above equation. The 

 equation really represents a parabola, but within certain limits 

 the spectrum may be considered normal. For a grating of 

 medium dispersion, the entire spectrum will be practically 

 normal ; but with one giving larger dispersion, as a Rowland 

 2 1 -foot, the scales of the middle and end differ by one and one- 

 half parts in a thousand at a distance of 3° from the axis. It 

 is necessary, therefore, that parabolic curved photographic 

 plates must be used, but within certain limits they may be 

 circular. In the experiments a small Rowland concave grating, 

 of 15,000 lines to the inch, radius of curvature one metre, and 

 ruled surface 1x2 inches, was employed, the photographic 

 plates being bent to the proper radius. The spectra of Sirius, 

 Capella, and Rigel obtained were 5 cm. long, and from O' i mm. 

 to I "5 mm. broad, and showed many lines. 



Thus with an exposure of forty minutes, the spectrum of 

 Sirius showed "16 hydrogen, H and K lines, and 15 other 

 distinct fine lines." Capella, with forty minutes' exposure, gave 

 F.G.h.H.K., and about fifty fine lines. It may be mentioned 

 that these experiments were made on the fifth floor of the 

 Physical Laboratory, ''subject to the jar of street-cars and city 

 traffic, as well as to dust and to the glare of electric lights,"' so 

 that the results were not obtained under the besi conditions. 



A Catalogue ok 636 Stars. — No. 4 of the Mittheihcngtn 

 der Hamburger Sternwarte contains a catalogue nf stars 

 observed by Herr W. Luther in the years 1885-92 with the 

 meridian circle of the Hamburg Observatory. The observations 

 in R.A. were made after the eye and ear method : those for 

 declination by bringing the stars between two horizontal wires. 

 The positions have all been reduced to the year 1885, and a 

 comparison is made with the catalogue of the Astronomische 

 Gessellschaft Zonen. 



THE PREPARATION OF MARINE 



ANIMALS AND PLANTS AS TRANSPARENT 



LANTERN-SLIDES. 



A T the request of the editor of Nature, I give an abridged 

 ■^ account of my essay in the volume of original researches 

 published to commemorate the establishment of the Sheffield 

 University College by Royal Charter in 1897. I shall confine my- 

 self mainly to the animals shown in the reproductions from four of 

 the series of photographs taken by Mr. J. E. Atkinson, of our 

 College, from some of my slides. Though on the whole these 

 reproductions show the general facts fairly well, much of the 

 minute detail is unavoidably lost, which is quite distinct when 

 the mounted animals or photographs are somewhat magnified. 



It is about eleven years ago that I first attempted to prepare 

 lantern-slides with marine animals. At first I did not mount 

 them in balsam, but very soon found that this is in almost every 

 case not only desirable but even essential, since they so readily 

 become mouldy, sometimes are attacked by mites, and are 

 often far too opaque. Some also scale off from the glass and 

 break to pieces, unless mounted. The success of the prepara- 

 tions depends almost as much on the proper mounting with 

 balsam as on anything else, and sometimes the only way to get 



NO. 1483, VOL. 57] 



excellent results is to mount several, and pick out the best, 

 which perhaps cannot be known until the specimens are finally 

 mounted in balsam. 



The methods necessary in mounting vary greatly in the 

 case of different animals. Often little else is wanted than to 

 arrange them properly on a lantern-glass, so that they touch it 

 more or less completely all over their under surface, and then to 

 drain and dry them. Many readily adhere round the drying 

 edges, before the central parts are dry ; and being thus fixed, 

 they do not shrink laterally on further drying, but merely become 

 thinner. On finally drying completely they may partially scale 

 off, and it may be desirable to gum them down in one or more 

 places, lest they should become loose when mounted in the 

 balsam. There are a few animals that will not adhere at all to 

 the glass, and yet shrink greatly. This circumstance has so far 

 prevented me from making satisfactory slides of Actinia. I 

 have succeeded with every other group. 



Few animals are more easy to prepare than small flat fish like 

 soles and dabs, 2 or 2^ inches long. These are killed by putting 

 them into dilute alcohol, and arranged on the glass as soon as- 

 dead, whilst still limp. The chief matter is to arrange out the 

 fins neatly. These soon dry, and adhere well ; but in order 

 that the side near the glass may keep flat, it is desirable or> 

 further drying to adopt a plan which I find most useful in many 

 other cases. Very few, if any, animals will adhere in an objec- 

 tionable manner to thin paper soaked with bees- wax, and, 

 having laid such over the animal, pressure can be applied. 

 What is wanted is that this pressure should be fairly uniform, 

 and not merely on the thick parts. This is easily done by 

 having a stout lantern-glass covered by two or three thicknesses 



Fig. I. — Priapubis in natural state. 



of fine thin flannel, which is pressed down by a smaller or larger 

 weight, so regulated as not to crush or distort the animal, but 

 rather to retain as much as possible the natural shape and show 

 the internal structure. The animal then dries through this- 

 flannel, and at the same time keeps sufficiently flat on the glass. 

 Finally, any specially high parts can be pressed down by using 

 a flat glass without flannel and a heavier weight. 



A considerable variety of marine worms can be made into 

 most excellent transparent slides, showing not only their general 

 shape and colour, but also much of their internal structure. 

 Sabella may be named as a specially good example. Such 

 animals should be killed by keeping them for a shod time in 

 dilute alcohol. The aim should be to dry them before partial 

 decomposition sets in and destroys the small blood-vessels. If 

 all goes on well it is possible to dry and permanently preserve 

 such worms as Nereis, so as to show not only the chief blood- 

 vessels but even the smallest branches, and the blood may retain 

 its red colour for years without any apparent change. 



As an example of an animal mounted without staining, I give 

 in Fig. I a reproduction from Priapuhis. It should be killed 

 by putting it into fresh water, and left in it so long that the body 

 just begins to get limp. It can then be easily arranged on the 

 glass, and adheres fairly well without lateral contraction. If 

 mounted at once or previously kept in alcohol the body is too 

 hard and will not adhere to the glass, and on drying contracts 

 so much laterally as to become very unlike the living animal. 

 The internal anatomy and general structure are best seen by 

 cutting the animal open from end to end, and staining the whole 

 with Beale's carmine or Kleinberg's hsematoxylin. When thus 



