Fig. 1. 



350 PROCEEDINGS OF THE ACADEMY OF [1893- 



The construction of the heliometer is based upon the fact that while 

 an object appears single (Fig. 1) when viewed through two 

 object glasses of equal focal length placed side by side at the end of 

 a tube provided with a single eye piece, it will appear double 

 (Fig. 2) when the object glasses are displaced. As the two images 

 (Fig. 2) at the focus are separated from each other by a distance 

 equal to the distance of the centres of the object glasses, the dis- 

 placement of the latter as measured by a micrometer screw will give 

 the displacement of the angular diameter, (Fig. 2, 

 c d) of a celestial body. The angle subtended by 

 the apparent diameter of the sun, for example Fig. 

 2, c d, can then be determined by simply bringing 

 the two images of the sun in contact (Fig. 2) and 

 measuring the displacement necessary to accom- 

 plish this by a filar micrometer, the angular value 

 of the revolution of the screw being known. 

 A few years after Bouguer invented his heliometer which Lalande 

 used to determine the diameter of the sun, Dollond substituted for 

 the two object glasses the two halves of one object glass (Fig. 3, A B). 

 By means of this arrangement two images (Fig. 2, a b) are seen as in 



Bouguer's heliometer, each semi-lens giving 



an image except when the two semi-lenses 



are juxtaposed, the two images being then 



superimposed. By fixing the semi-lens A 



and making the semi-lens B(Fig. 3)slide over 



it, the image a (Fig. 2) due to the semi-lens 



A becomes also immovable. The distance 



through which the semi-lens B (Fig. 3) is 



moved in order to bring the image b (Fig. 



2) due to it, in contact tangentially with 



the image a and equal to the apparent 



ri g. 2. diameter sought, is measured by means of 



a micrometer screw as in Bouguer's instrument. The value of the 



angle under which the apparent diameter of the object (Fig 2, c d) 



is seen can be thus determined. 



In connection with the invention of the heliometer, it is interest- 

 ing as illustrating the influence of the progress of astronomy upon 

 physiology that the Observatory of Konigsberg, to which Helmholtz 

 was attached as assistant when a young man, possesses one of the 

 finest heliometers ever constructed : that by Fraunhofer. Doubtless 

 this instrument suggested to Helmholtz the invention of the ophthal- 



