650 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



which nature constitutes those images which are formed in optical instru- 

 ments or on the retina. Two images, and their relation to one another, 

 are considered. One of these is the image of the celestial object pre- 

 sented to the telescope or to the eye. The other, referred to as the con- 

 centration image, can be formed by the same identical light, but is an 

 entirely different image. The method of analysis which has proved to 

 be the most efficient in tracing out how images are actually formed by 

 nature is the analysis of the light, within any space occupied by a 

 uniform medium, into its constituent undulations of flat wavelets. It is 

 by the interferences of these undulations that the image is formed. The 

 author describes an apparatus for making experiments which will disclose 

 the imperfections of necessity existing in the images furnished by astro- 

 nomical telescopes, and which will indicate the causes of these imperfec- 

 tions and how they may be mitigated. He also discusses why it is that, 

 when a large and a small object exactly similar to each other are examined 

 with the same telescope, the large one will be seen satisfactorily, while 

 the small one, though of precisely the same shape, will, if small enough, 



Fig. 113. 



Pig. 114. 



appear when viewed through the telescope to be transformed into some- 

 thing unlike itself. 



He appeals to microscopical experiments * in illustration of his 

 subject, and describes certain observations on the proboscis of a blow- 

 fly. Fig. 113 shows an arrangement of hairs on a certain specimen. A 

 small triangular patch of bright light happened to be shut in between 

 three of the hairs or bristles which grow near the base of the proboscis. 

 These hairs had been pressed in the mounting of the specimen so that 

 they lay near to one another and nearly in a plane perpendicular to the 

 optic axis. Another, and thinner hair, which we may calk the canal, 

 divided the triangle of light into a smaller triangle below and a quadri- 

 lateral space above the canal. Within this quadrilateral was seen about 

 half of the base of another small hair, presenting the semicircular 

 appearance in fig. 113, and being somewhat darker on the right-hand part 

 of the semicircle than on the left. The rest of this hair lay outside the 

 triangle of light to which attention is being called. This object was 

 examined through one of Zeiss' 24 mm. apochromatics, over which an 

 iris diaphragm had been fitted to enable the observer to diminish its 

 aperture to any desired extent. The succession of appearances while 



* Phil. Mag., xvi. (1908) pp. 976-7. 



