OBSERVATION OF PHENOMENA. 315 



of the most perfectly constructed and complex arrange- 

 ment of apparatus, used under the most favourable con- 

 ditions, can they be detected at all. In microscopic 

 manipulation we require to adjust the lenses to the exact 

 focal distance, in order to see the lines on test-objects. In 

 using a spectroscope we have to exclude extraneous light, 

 and to separate as far as possible the compound lines with- 

 out too much enfeebling them, and so on. 



Descriptions of the methods of preparing and using all 

 the different substances, the modes of making particular 

 experiments, the manipulation and precautions requisite 

 in using different kinds of apparatus, &c., may be found 

 in all the text-books of physics and chemistry. How to 

 work with a microscope requires a treatise to itself, simi- 

 larly also with a spectroscope and with the instruments 

 and materials of each particular science and art. Some 

 books are entirely devoted to descriptions of the modes of 

 manipulation ; for instance, amongst others may be men- 

 tioned Faraday's 'Chemical Manipulation,' Williams's 

 'Hand-book of Chemical Manipulation,' Kohlrausch's 

 ' Physical Measurements,' Latimer Clark's ' Electric Mea- 

 surement,' &c. 



CHAPTER XXXIV. 



OBSERVATION OF PHENOMENA. USE OF THE SENSES IN 

 SCIENTIFIC RESEARCH. 



THE term ' observation ' is usually applied to the direction 

 of our senses and perceptive powers to objects or phe- 

 nomena, especially external ones. In order to observe a 

 thing consciously we usually require to employ both our 

 senses and perceptive abilities ; thus we may be looking at 



