IMPERFECTIONS OF OUR OBSERVATIONS. 321 



source of other ideas. As the mind cannot be simul- 

 taneously occupied by two ideas, one of which is so vivid 

 as to occupy the whole of its attention, so an idea arising 

 from a strong sensation usually shuts out all power of 

 observing others. 



That which one man misses in science, another per- 

 ceives. This is often the case ; for example, I discovered 

 the sudden elongation of an iron wire at a particular tem- 

 perature whilst under longitudinal strain during the act of 

 cooling from a red heat ; but Professor Barrett by repeating 

 the experiment in a darkened room made an additional 

 observation which I had missed, viz., that at the moment of 

 elongation the wire suddenly evolved heat, and exhibited 

 a visible and conspicuous momentary glow of redness. 



As the great bulk of material phenomena are extremely 

 minute, and our faculties of observation are exceedingly 

 feeble, we observe only a small proportion of the effects, 

 and those only the larger ones, which actually occur. Non- 

 observation, therefore, does not prove non-existence, 

 except in those cases in which it is certain that the objects 

 if present would have been observed. As our faculties are 

 powerless to appreciate perfect accuracy, and scientific 

 instruments are more or less imperfect, the results we 

 observe are always probable or approximate only. Many 

 phenomena are also accompanied by others which cannot 

 be separated. For example, current electricity is always 

 accompanied by magnetism ; chemical affinity is nearly 

 alwavs attended by evolution of heat ; the action of 

 gravity is a concomitant of every phenomenon ; and so on. 

 Some phenomena also are masked by others ; for instance, 

 positive and negative electricity often disguise each 

 other. Sometimes we fail to observe, because we have not 

 used the proper means to produce the greatest degree of 

 effect, or we have not obtained a sufficiently conspicuous 



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