300 



NATURE 



\yuly 1 8, 1878 



arranging one part so as to work in with another, or 

 rather to lead up to it, is a good one. The aim has been, 

 not to simphfy terms, which has often been attempted 

 with varying success, but to reduce as far as possible 

 the difficulty always attending a clear understanding 

 of the meaning of the terms, and indeed to simplify 

 the whole system of teaching. "To this end," the 

 author says in his preface, "two charts of the natural 

 orders are given, in which the diagnostic characters 

 are reduced to a minimum, those which are most 

 easily observed having been chosen as far as pos- 

 sible in preference to the more minute, while all the 

 exceptions have been indicated in an appendix. It is 

 hoped that .in this way, the student being familiarised 

 with all the exceptions likely to be met with in this 

 country, some of the difficulties attending a practical 

 study of botany will be removed." The three diagrams 

 of scarlet geranium, daisy and dandelion, and narcissus 

 will be found very useful, as each part of the plant is very 

 distinctly named on the plate itself and is furthermore 

 minutely described in two and a half pages of letterpress. 

 The glossary with the Latin terms accented will be a 

 great help to a young beginner and the interleaving of 

 this part is a good point. Akogether we think the book is 

 very satisfactory. We should, however, have preferred to 

 see the sixty schedules placed at the end of the book 

 rather than in the middle. Placed where they are, one is 

 led to suppose there is no further matter beyond them, 

 which is not the case, the charts and a very useful " Floral 

 Calendar " being placed at the end. 



GrundzUge der Electricitiitslehre. Zehn Vorlesungen 

 von Dr. W. v. Beetz. (Stuttgart : Meyer and Zeller.) 



Dr. von Beetz has published a series of lectures de- 

 livered to the members of the medical association in 

 Munich. These lectures do not pretend to contain an 

 exhaustive treatment of the subject. They are meant to 

 illustrate the fundamental principles of the science by a 

 series of well-devised experiments, and they amply fulfil 

 the object for which they have been written. The little 

 book contains much matter in a small space, and is 

 throughout clear and to the point. It will be useful to a 

 wide class of readers especially as an introduction to 

 more detailed treatises. Objections might be raised 

 against some incidental and more speculative remarks ; 

 but these are very few in number, and do not affect the 

 chief aim of the book. A. S. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters a% 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts. "l 



Measuring Scales for Pocket Spectroscopes 



In using small specti-oscopes, such as miniature pocket in- 

 struments, to examine colom-ed flames, and to discover to what 

 particular substances they owe the characters of their radiation, 

 the power of such discrimination which these instruments possess 

 is for many reasons very limited and circumscribed if they are 

 unprovided with some description of dark-field illuminated scale 

 of regular divisions. I liave found the following description of 

 measuring scale, when adapted to such small spectroscopes, 

 answer ordinary purposes of recording bright-line positions with 

 them very well, although it was for viewing faint lines in 

 auroral spectra that it was originally devised, for which I have 

 not yet had an opportunity to test its suitability with the same 

 success. 



The circular disc A is a piece of copper foil in which a fine 



slit a is punched, and oblique to it a row of twenty holes is 

 punctured, five on the left and fifteen on the 

 right of the slit, the highest and lowest holes 

 of the row being level with its top and bottom 

 points. The actual size of the disc is just 

 half the size of this figure, and to puncture 

 the holes at equal distances apart of about 

 •jVth of an inch, either a dividing engine 

 must be used and a needle-point drawn along 

 the sloping face of a straight edge is pressed 

 down with the equal pressure of a weight 

 upon the disc laid on zinc or ivory, to puncture it ; or a 

 rack of fifteen or twenty of the finest sewing needles, side 

 by side, may be so fixed in fusible metal as to produce the 

 whole row of punctures by a single pressxire. But the first 

 method, even with a roughly made dividing engine for the pur- 

 pose, I have found the easiest and the most successful plan. 

 The disc takes the place of the jaws of a pocket spectroscope, 

 being dropped into a recess where it is covered by a glass plate 

 and held in its place by a brass ring or perforated cap screwed 

 upon the end of the spectroscope. When viewed through the 

 prisms by sodium light it is seen magnified by the eye-lens, and 

 the punctiures form a scale of bright yellow points to the right 

 and left of the yellow sodium line produced by the slit. 



With sufficiently large punctures it is probable that the mono- 

 chromatic yellow -green auroral ray would render 'the punctured 

 scale visible in the same way that the sodium light does, so as 

 to supply a measuring scale on which other spectral rays of the 

 aurora's light besides the greenish one produced by the slit may 

 be observed and recorded in their actual positions of distance 

 from that leading line. Even the chief green ray of the solar 

 corona in a total eclipse would not improbably illuminate the 

 oblique scale sufficiently to allow the positions of other rays 

 occurring in its spectrum to be recognised and mapped with ease 

 and with considerable accuracy with a pocket spectroscope. 



The object of inclining the row of punctures obliquely from a 

 horizontal line is that other coloured images of it besides the 

 principal or brightest one chosen for reference may not mix with 

 and confuse its divisions. There is no means of varying the 

 \^ idth of the slit in the arrangement, and I have not succeeded 

 in obtaining microscopic scale photographs on glass sufficiently 

 dense and opaque to replace the metallic punctured scales, and 

 the focus of the eye lens for the yellow sodium points is not 

 exactly the same as for very refrangible blue lines that coincide 

 with them in position in the field of view ; but an assortment of 

 discs can be used and may be placed at pleasure in the cell, and 

 the objection of the unequal focus is at least removable by using 

 an achromatic lens. With these drawbacks to its use, however, 

 the punctured scale has one essential advantage over laterally 

 reflected ones, that its relation to the spectrum which it is used, 

 to measure always preserves an invariably fixed adjustment. 



The presence of sodium light is so easily supplied where it is 

 wanting that there are very few conditions (even if another 

 bright spectral line cannot be chosen) in which the punctured 

 scale is not available. The wave-length curve for each spec- 

 troscope is then easily constructed from observations of known 

 elementaiy metallic lines in flame spectra in which the sodium 

 line is always available for a line of reference, I have obtained 

 this curve for both a punctured scale spectroscope and also for a 

 Browning's miniature spectroscope with a reflected scale, and 

 have examined several commonly occurring flames in furnace and 

 other fires with the result of detecting in them some metallic 

 spectra. Lithium is thus very often found, sometimes almost as 

 bright as the sodium line, in coke furnace fires ; and I have 

 frequently observed in ordinary house fires a flame of rich blue 

 colour with a very characteristic spectrum, which I now recognise 

 by measurements as agreeing with that of copper chloride ob- 

 tained by moistening copper foil with hydrochloric acid in a 

 Bunsen-flame. 



The accompanying figure (projected as nearly as the small size 

 of the drawing permits, in tenth-metre wave-lengths) shows 

 the appearance of the natural and artificial spectra, and their 

 close resemblance, showing very clearly that some compound of 

 copper is the cause of the very brilliant blue colour of the 

 natural flame. Its finest exhibition in house-fires (where it quite 

 filled a grate) arose from blazing wood-logs, which were described 

 to me as broken -up ship-timber, in which traces of copper 

 chloride might reasonably be expected to occur, and it is in the 

 wood of fires that its hyacinth-blue flame has usually attracted 

 my attention ; but the same spectrum with three neat pairs of 



