NOTES. 



H3 



been reached. But, if the intensity of the 

 stimulus be now as gradually diminished, 

 the eye will continue to perceive it till it 

 has fallen to one third or one fourth of 

 the original minimum. In producing the 

 initial sensation a certain amount of light 

 has, so to speak, been wasted in putting 

 the machinery in motion. Further, if the 

 eye has been carefully shielded from the 

 light for some minutes before performing 

 the experiment, it will be capable of per- 

 ceiving light which is fifty or even one 

 hundred times less intense than that re- 

 quired to produce a luminous sensation. 

 This enormous difference is equally manifest- 

 ed whether monochromatic or white light 

 be employed. Now, if we apply a similar 

 test to the sensation of color, we find that 

 for the chromatic as for the luminous stimu- 

 lus a certain minimum is needed to produce 

 the sensation, which still continues to be ex- 

 cited when the intensity of the stimulus is 

 progressively diminished. So far, the two 

 sensations, of light and color, obey the same 

 law. But if we proceed to compare the 

 sensitiveness of the eye in full activity with 

 that of the eye which has been allowed a 

 period of absolute rest, we no longer find 

 any such increase in its susceptibility to 

 the chromatic stimulus as was observed in 

 the case of light. This result is altogether 

 opposed to the current opinion that the 

 sensation excited by white light is really 

 a resultant of the simultaneous development 

 of several determinate color sensations ; it 

 shows, on the contrary, that the sensation 

 of light is altogether independent of that of 

 color, and really a simpler kind of reaction 

 on the part of the visual apparatus. 



" Oil on the Troubled Waters." The 

 fishermen of the Shetland Isles, as we learn 

 from a writer in " Chambers's Journal," are 

 wont, when in utmost peril during a storm, 

 to throw oil on the waters to still them. 

 They crush in their hands the livers of any 

 ling or cod they may have caught, and keep 

 throwing them astern and around them. 

 " The effect," we are told, " is magical. 

 The waves are not lessened in size ; but 

 they no longer break, and it is only from 

 their breaking close to the boat and so 

 being dashed in upon her and filling her 

 that there is danger. The rapidity with 

 which the oil spreads over a considerable 



space of sea around is marvelous, and 

 scarcely to be credited except by one who 

 has witnessed the phenomenon." An ex- 

 pedient so simple might often be of invalu- 

 able service in saving life and property. 

 The difficulty and peril, for instance, of 

 launching a boat from a sinking ship in a 

 storm are mostly caused by the wind break- 

 ing the waves over the boat and filling her 

 or dashing her against the vessel's side. 

 " The danger of such a mishap would un- 

 questionably be greatly lessened by throw- 

 ing overboard some oil, which ought always 

 to be kept handy. Boats also going from 

 one ship to the assistance of another in dis- 

 tress, and life-boats on their way to a wreck, 

 and boarding it, might often with very great 

 advantage use a little oil, if its effects were 

 only better known. Another case in which 

 oil might be of the greatest service is when 

 a man accidentally falls or is washed over- 

 board. Life-buoys are thrown into the sea, 

 the ship is brought to as quickly as possible, 

 boats are lowered and a search made ; but, 

 before all this can be done, the vessel has 

 run a considerable distance, and, although 

 the poor struggler in the water may be a 

 good swimmer and able to keep afloat for 

 some time, the great difficulty is to find the 

 exact spot where he is to be sought for. A 

 life-buoy or a man's head is a small object 

 to descry among heaving waves and white 

 foam. If life-buoys were constructed so as 

 to contain a small portion of oil in a little 

 receptacle or India-rubber bag attached to 

 them, to be punctured with a knife before 

 being thrown overboard, the effect would 

 be not only to prevent the sea from break- 

 ing over the castaway, so making it easier 

 for him to keep afloat, but would indicate 

 to the searchers almost the exact spot 

 where to look for him." 



NOTES. 



The commonly received theory of dew is 

 that it results from the condensation of the 

 moisture of the air by contact with surfaces 

 of a lower temperature. This theory is re- 

 jected by Professor Stockbridge, of the Mas- 

 sachusetts Agricultural College. He holds 

 dew to be the vapor from the soil condensed 

 by the cooler air, and states as follows the 

 results of his experiments: 1. The vapor of 

 the soil is much warmer at night than the 

 air, and would be condensed by it. 2. Va- 



