cause. And now I want to show you what beautiful results he obtained. We may represent what the eye saw by the diagram of Fig. 34. The time when the light is steady is represented by the horizontal lines like CD and FG: the eclipses by the sudden drops ABC and DEF. But the first thing found by the selenium photometer was that in the middle of the straight portions (or what had been thought to be straight portions) there was another little drop so small that the eye had overlooked it. What does this mean?

Fig. 34. The big drop ABC means that the dark body comes in front of the bright one: the little drop means that the bright one comes in front of the dark. If it were entirely dark, that would of course make no difference: and since there is a noticeable difference, it follows that the second body cannot be entirely dark, but must be giving off some light which is cut off when the brighter body comes in front. This, then, was the first thing found out; but there is more to come. Not only is there a little dip in the middle of each supposed straight portion, but the remaining parts of those portions were found to slope slightly in opposite directions, as shown in Fig. 35. This means that even when the two bodies are clear of one another, and no eclipse is taking place, the light is still changing in some way. Mr. Stebbins was able to give the