178 ELEMENTS OF LABORATORY WORK 



often seriously inaccurate in themselves, and conduce to a feeling of 

 certainty when the training of observation has barely commenced. 

 Keenness and faculty of research is thus suppressed, when the aim of 

 even the most elementary work should be to encourage it. 



9. Observations of Radiation. It is essential for many experiments 

 with light, that a portion of the laboratory should be capable of being 

 darkened at will. A room with dark-blinds to the windows is valuable 

 for advanced work, but for beginners and large classes this is evidently 

 not desirable. It is more consistent with good order to have certain 

 portions of benches fitted with supports carrying velvet curtains, 

 which give access to the instruments. The light may be prevented 

 from entering above by placing dusters or cloths at the top as re- 

 quired. Such shielded enclosures serve also for working' with mirror 

 galvanometers and electrometers. 



It is of great importance that prisms, lenses, gratings, &c., should 

 be handled with care, lest scratches should injure their surface. 



In working with the spectroscope, the necessary exclusion from 

 the prism or grating of rays which do not come from the slit is 

 readily obtnined by throwing over 1he instrument a piece of velvet. 

 All spectroscopes should be capable of being used as spectrometers. 

 Hence the central table should be moveable, independently of the 

 larger table. The latter table should be as large as possible, other- 

 wise the movements are cramped, and readings become troublesome. 

 A valuable exercise is contained in the complete adjustment and 

 levelling of this instrument. It should be occasionally put out of 

 adjustment with this in view. 



In order to obtain light of a particular wave-length, a Bunsen 

 flame should be used, and the requisite material, placed upon a piece 

 of clean platinum foil, should be supported in the lower part of the 

 flame towards the edge. The most convenient light is the yellow 

 light obtained in this way from sodium chloride or carbonate. 



Most of the formulae connected with reflexion and refraction of 

 light have been omitted as beyond the scope of this book, and pro- 

 perly belonging to a more advanced stage of inquiry. The merely 

 qualitative study of light has been introduced with a view to prepare 

 the ground for observations of physical changes from the point of 

 view of energy. The rapid growth of complexity along with progress 

 is seen, perhaps too clearly, in the sections on f Eadiation,' and in the 

 delicacy of the apparatus required. The optical bench is an involved 

 and expensive piece of apparatus. For elementary work the outlay 

 would be very disproportionate to its utility ; but for advanced work 



