716 



THE POPULAR SCIENCE MONTHLY. 



effective, a filtering apparatus must either 

 remove or destroy any micro-organisms con- 

 tained in the water. 



Color Photography. At a recent soiree 

 of the Royal Society, in London, Dr. Joly, of 

 Dublin, exhibited some photographic trans- 

 parencies upon glass plates representing va- 

 rious objects in their natural colors. The 

 subjects photographed were especially chosen 

 because of variety of color and delicate 

 shading, and were reproduced with great 

 naturalness and fidelity. The results were 

 accomplished by the use of a finely ruled 

 glass plate, two hundred to three hundred 

 lines to the inch, each three lines being a 

 complete color series, consisting of an orange- 

 yellow line, a greenish-yellow line, and a 

 blue-violet line, these colors being repeated 

 over and over again. The lines are ruled 

 with colored inks, made up of gum and 

 gelatin mixed in certain proportions, on a 

 gelatin-coated plate. The plate to be ex- 

 posed is placed in contact with this color- 

 screen, and only exposed to light which has 

 passed through the latter ; an extra-long 

 exposure is necessary, owing to the partial 

 opacity of the color-screen. The plate is 

 then developed in the ordinary way. The 

 color-screen is now again placed against the 

 negative, and when the two are held up to 

 the light, if the color-screen is placed just 

 as it was when the exposure took place, an 

 accurately colored reproduction of the origi- 

 nal scene appears. The process is so simple 

 and inexpensive that it will probably come 

 rapidly into general use. 



The Yalne of Object Lessons. In a re- 

 cent educational circular we find the follow- 

 ing on object teaching : " To sum up the 

 main value of object teaching, there are 

 three principal uses : The first and most im- 

 portant is to teach the children to observe, 

 compare, and contrast ; the second is to im- 

 part information ; and the third is to re-en- 

 force the other two by making the results of 

 them the basis for instruction in language, 

 drawing, number, modeling, and other handi- 

 work. There are, however, other important 

 uses of good object teaching. It makes the 

 lives of the children more happy and inter- 

 esting by opening up an easily accessible 

 and attractive field for the exercise of brain, 



hand, and eye; it gives the children an op- 

 portunity of learning the simplest natural 

 facts ; and directs their attention to external 

 objects, making their education less bookish. 

 It further develops a love of Mature and an 

 interest in living things, and corrects the 

 tendency, which exists in many children, to 

 destructiveness and thoughtless unkindness 

 to animals, and shows the ignorance and 

 cruelty of such conduct. The value of the 

 services which many animals render to man 

 should be dwelt upon, and the importance 

 of kindly treating them and preserving them 

 should be pointed out. By these means, 

 and in other ways, good object teaching may 

 lay the foundation for the right direction of 

 the activity and intelligence of the children 

 throughout the whole school." 



Sunlight and Pictures. The question of 

 preventing or mitigating the fading of pic- 

 tures and pigments has been attacked in 

 earnest and in a practical way by Captain 

 W. de W. Abney, who finds that fading in 

 the course of time is one of the inevitable 

 effects of the operation of ordinary sunlight. 

 Pictures can not well be taken from the 

 light, so the next best thing is to discover 

 which of the solar rays do the most damage, 

 and to mitigate their effects as far as pos- 

 sible. The violet rays prove to be most ac- 

 tive in producing fading. If we can elimi- 

 nate the majority of these rays from white 

 light without appreciably altering the fresh- 

 ness of the colors viewed in such light, we 

 shall practically have prolonged the life of 

 a picture. A variety of experiments made 

 with different pigments tell us that the loss 

 of the violet of the spectrum is practically 

 no loss at all. Even with white light the 

 loss is unnoticeable. If we form a patch of 

 light composed of all the colors except the 

 violet, we shall notice but little change from 

 the pure white that is alongside of it. The 

 case becomes simpler yet when we find that 

 the blue-green light and the yellow light of 

 the spectrum superposed give substantially 

 white. A blue-green glass and a yellow 

 glass interposed against the sunlight practi- 

 cally cut off all the violet, while they give 

 passage to the rays that form white. Cap- 

 tain Abney therefore solves his problem by 

 using glasses of these colors for the window- 

 glazing of hjs gallery. Making the windows 



