498 THE POPULAR SCIENCE MONTHLY 



images on the plate to move, slowly with short-focus and rapidly with 

 long-focus cameras, tlius drawing them out into trails. "With the 

 longer instrument here described, an eight-minute exposure would in 

 general be no more effective than one of four minutes. The most 

 successful instrument for the search in question must compromise be- 

 tween the advantage of long focus in reducing sky density, and the 

 disadvantage of long focus in producing long trails. Shorter ex- 

 posures, giving shorter trails, may be provided by increasing the diam- 

 eter of the lens, but this in turn means greater unavoidable optical 

 aberrations in the outer areas of the region photographed, which is a 

 reduction in efficiency. In this as in all instruments, extensive 

 experience and good judgment must combine to decide upon the best 

 compromise-proportions. 



Professor Pickering, of Harvard, and Mr. Abbot, of the Smith- 

 sonian Institution, used such cameras at the total solar eclipse of 

 1900. The latter observer was favored with good conditions, in North 

 Carolina, and he secured one photograph of a considerable area sur- 

 rounding the eclipsed sun. Quite a number of the stars known to 

 exist in this region were photographed; but in the absence of a dupli- 

 cate photograph of the same region, he could not decide whether 

 certain apparent images on the plate were due to unknown planets, or 

 were defects such as always exist in photographic films. 



At the eclipse of 1901, in Sumatra, Mr. Abbot, of the Smithsonian 

 Expedition, and Mr. Perrine, in charge of the Crocker Expedition from 

 the Lick Observatory, were prepared, with four cameras each, to secure 

 duplicate photographs covering a large area extending east and west 

 from the sun. Conditions were unfortunately against the success of 

 Mr. Abbot's plans, but thin clouds at the time of the eclipse let 25 

 per cent, of the light come through to Mr. Perrine's photographic 

 plates. The area covered in duplicate was 6° x 38°, extending along 

 the direction of the sun's equator, with the sun in the center of the 

 region. The plates recorded 170 well-known stars; and all apparent 

 images not of ordinary stars were proved by the duplicate plates to 

 be defects in the films. In two thirds of the area stars down to the 

 eighth magnitude and many fainter ones were recorded; and in one 

 third the area, covered with thicker clouds, stars were recorded down 

 to the fifth and sixth magnitudes. 



At the eclipse of 1905 Mr, Crocker made it possible for me to 

 organize expeditions to Labrador, Spain and Egypt, each equipped with 

 four intramercurial cameras, in addition to apparatus for other lines 

 of research. The details of the twelve cameras were planned by Dr. 

 Perrine, the instruments were constructed under his supervision, and 

 any photographic plates obtained with them at the three stations 

 were to be assigned to him to examine for possible intramercurial- 

 planet images. The Labrador group of four cameras, mounted at the 



