May -20, 1SS5.] 



♦ KNOWLEDGE ♦ 



453 



I remark, en passant, that the mere resolution of a 

 specially hii;h band seems to have been erroneously re- 

 gardetl by Herr Nobert and others as a sutlicient test of the 

 qusJity of an objective; whertas it can be demonstrated 

 that the fact alone that an objective resoU cs a given band 

 proves only that the particular zone of aperture which is 

 utilised in such resohition is free from aberration ; and this 

 may obtain in an objective in which nearly the wliole of 

 the other portion of the aperture is so atlected by aberra- 

 tion that the objective, as a whole, must be regarded as of 

 inferior quality. 



With reference to the interference plates and diflVaction 

 gratings ruled by Herr Nobert, I cannot speak from per- 

 sonal experience with them. He claimed for them that 

 they were the real tests of the accuracy of his ruling 

 machine, as regards exact equidistance of the divisions, 

 parallelism of the lines, equality in thickness, and symmetry 

 of angles of the actual rulings ; and as he had to face many 

 sceptical critics, I think it may fairly be assumed that his 

 claims were admitted, inasmuch as they were not refuted. 

 I shall have to direct your attention to the special means 

 employed by Herr Xobert to secure the strict equidistance 

 of the divisions required for the diffraction gratings, where 

 lines of one Paris inch in length were ruled over a breadth 

 of one Paris inch, at the rate of 1,200 to 12,000 to the 

 Paris inch — that is to say, the whole space was covered 

 by lines of these degrees of fineness. He states that 

 " these gratings show spectra of such perfection, that in 

 the wave-lengths of the Frauenhofer lines, as established 

 by Prof. Listing, even the ioou'oo ttt; o^ * millimetre is 

 made apparent ;" and that the absolute measure of his 

 divisions was " founded on a standard accurately corre- 

 sponding with the original Bessel standard-measure made 

 by the distinguished Berlin mechanician, Baumann." 



Herr Nobert appears formerly to have held that Frauen- 

 hofer's formula based on the constitution of light (to which 

 I have alluded) would present an absolute bar to the reso- 

 lution of lines finer than ^7,\jtj °^ ^ Paris line (^Triirr) to the 

 English inch) ; Dr. Woodward's photogi-aphs, however, dis- 

 proved this theory to his complete satisfaction. A sugges- 

 tion was also made by an adverse critic, that possibly the 

 higher bands, which had not been resolved, did not really 

 contain the lines alleged by Nobert ; to which Nobert 

 answered by providing an interference - plate * which 

 "proved indirectly (= --L^th of a Paris line = TnriVTT^^ 

 of an English inch) the existence of all the lines up to 

 0"'-00012o by the spectra they showed. With this plate 

 was also proved, empirically, the inference from the un- 

 dulatory theory of the disappearance of the colours, when 

 the distance of the lines is less than the smallest wave- 

 length in the medium used." 



{To be continued.) 



THE YOUNG ELECTRICIAN. 

 By W. Slingo. 



(Continued from p. 407.) 

 WOOD-BOEIXG TOOLS. 



EX. LVII. — The young electrician will do well to have 

 always by him a few gimlets and bradawls of various 

 sizes. They are too well-known to require any description. 

 It may, however, ha observed that when he desires to bore 

 a relatively larf;e hole, and is restricted to gimlets, it is 

 best to first make a small hole, and increase it by subse- 



* Poggendorff's " Annalen," Ixxxv., p. 83. 



quently introducing a larger tool. Otherwise there will be 

 considerable risk of splitting the wooil. Tlie fact that 

 gimlets taper towards the pidut increases the risk of 

 splitting, more especially in the case of soft wood. 



A bradawl is not, in the true sense of the word, a 

 boring-tool, as it simply squeezes the wood away as it 

 enters. Nor is a giniblet very much better ; it r<rtainly 

 removes some of the wood, but a great portion of its work 

 consists also in squeezing. A giniblet is only a cutting- 

 tool in a very limited sense ; more frequently it lenrs the 

 wood away — a fact which is often responsible for the 

 splitting of the wood. 



Ex. LVIII. — To turn to tools which cut at the same 

 time they bore a hole, Fig. 30 illustrates a few forms of 

 what are known as bits. 



The most frequently used are those lettered a and b, the 

 former being called a centrebit, and the latter a spoonbit. 

 They are, of course, made in a great number of sizes, 

 suitable for as many different sized holes. 



The centrebit has usually a long square shank, the 

 lower portion being flattened out approximately to the 

 shape shown, the extremity taking the form of a point, 

 which lies in the axis of rotation. On the left side (of the 

 drawing) is the cutting edge, which consists of a small 

 portion of the metal bent forward and thinned into a 

 species of knife edge. On the other side, at a distance from 

 the centre equal to that of the remote extremity of the 

 cutting edge, is a sharp and pointed prong, the metal 

 between it and the centreing-point being well cut away. 

 The function of the prong is to assist in maintainiog the 

 position of the tool. A hole cut by the centrebit should 

 be parallel throughout, and straight. 



The spoonbit differs from the centrebit in that the 

 cutting edge is vertical. In section it partakes of a semi- 

 circular form, the radius from which the inner side is 

 struck being, however, smaller than that of the outer. 

 The end of the tool is rounded. For long holes, which are 

 required to be cut straight, the spoonbit is preferable to 

 that previously described, on account of parallelism exist- 

 ting between the sides or edges of the bit and the hole so 

 far as it is bored. 



A bit of a character similar to that of the spoonbit, but 

 less frequently used, is that known as the rimer or taper- 

 bit. The illustration (Fig. oOc) explains its shape 

 sufficiently well to ob\-iate the necessity of any further 

 description. The tool is used for cutting a taper hole or 

 opening out a hr)le in thin material (where the use of either 

 of the other bit.5 would probably resiilt in spciling the 

 board). 



When screws are used in the wood, it is in most cases 



