EECEEATIYE SCIENCE. 



335 



80,000 ia the inch is about equally difficult. 

 It is interesting to observe here that the best 

 authorities are agreed in estimating the lines 

 on the most finely-marked of the Diatomacese 

 yet resolved, at about the same figure, 80,000 

 to 85,000 in the English inch ; and although it 

 may be rash, in view of the strides made during 

 the last few years, to say it seems as if it 

 might have been written with regard to these, 



" Thus far shalt thou go, and no further," 

 yet it does appear as if the probabilities 

 pointed that way. Eays of light won't go 

 through everything ; object-glasses have al- 

 ready been made with angles of aperture that 

 cannot be exceeded. 



As tests of aperture, Nobert's Lines are 

 the best, but since these lines may be seen 

 by glasses of large aperture, yet out in the 

 centring, faulty in the fitting, it is evident 



that something more is required than the 

 showing these well, before an object-glass can 

 be pronounced " first-rate." And, after all, the 

 showing the markings on fine-lined objects 

 is only one of the purposes of a microscope ; 

 perhaps it is not too much to call it, for real 

 service, one of the least. If information be 

 required on structure, on the component tis- 

 sues of an object under examination, to be 

 able to look through some depth of substance 

 is absolutely essential ; and it is satisfactory 

 to observe that the efforts of makers are now 

 being directed to the production, at reason- 

 able prices, of first-rate object-glasses, with 

 moderate angular aperture, rather than to 

 such as possess little working value beyond 

 showing " Noherfs Test-Lines," and other 

 fine-lined objects. 



TiTFFEN Wj2ST, F.L.S. 



PRICTICAL PHOTOaRAPHY. 



Photogeapht has now fairly taken its place 

 as one of the useful arts ; the purposes to which 

 it is applied are almost innumerable, and it 

 would be difficult to name an art or a science 

 which is not largely indebted to it. The 

 chemistry of photography is now so well 

 understood, that pictures produced by it, if 

 prepared with proper care, and according to 

 approved formulae, may be considered as per- 

 manent as drawings or paintings. 



There appears now to be a slight pause in 

 the progress of photography. Its general 

 principles are well known. Thousands of able 

 chemists and enthusiastic photographers have 

 lent their aid to bring it to its present per- 

 fection. IN'o great discoveries have been 

 lately made, and the energies of all are occu- 

 pied in perfecting and simplifying the pro- 

 cesses already known. Shall we continue to 

 wait until some grand discovery so far eclipses 

 our present performances as to make useless 

 our present apparatus and materials ? Or shall 



we not rather bring into general use photo- 

 graphy as it now is, and avail ourselves of its 

 assistance for a variety of purposes ? To the 

 artist, the geologist, the botanist, the mecha- 

 nist, etc., how valuable is photography. Who 

 can copy a landscape with such fidelity, a 

 building with such fine perspective, a model, 

 a machine, a fossil, a plant, a map, a drawing, 

 a manuscript, a coin, with the same accuracy 

 as the camera? Once obtain a negative, and 

 it may be reproduced, like an engraving, to 

 any extent. Every lover of science sliould 

 understand photography sufficiently to enable 

 him to use it for these, and a variety of pur- 

 poses, which are almost of daily occurrence, 

 and it is for the use of such this sketch is 

 intended; not to ieSiGlx photographers, but to 

 endeavour to show that any one with mode- 

 rate patience, and the use of his fingers, may 

 be enabled, in a very short time and at a 

 very small cost, to learn sufficient of the art 

 I to practise it with success, and to make use 



