62 E. M. NELSON ON BINOCULARS. 



in metal. Note, hyperstereoscopism can be easily produced by 

 the use of a too large stop at the back of the condenser for dark 

 ground illumination; this, therefore, should be carefully 

 avoided. 



In conclusion it must be remembered that duplex illumina- 

 tion alters the conditions, and, to a certain extent, upsets the 

 rigid theory, because an axial pencil is sent excentrically through 

 each half of the lens (Fig. 13). 



Explanation of some op the Figures. 



Fig. 4. — A stop to be placed at the back of the objective. 

 A separate one is required for each lens. 



,, 12. — A stop to be placed at the back of the condenser. 

 A special one is required for each objective. 



„ 11. — The white dots show the spectra, the ring in the 

 centre represents the dioptric beam. 



„ 13. — The rings in the centre of each circle represent the 

 dioptric beams. The white dots are spectra. 



,, 7. — Parallactic drawings of two truncated square pyra- 

 mids, the displacement is greatly exaggerated. 



,, 8. — A square parallelopiped drawn in perspective. 



,, 9. — The same drawn by parallactic displacement. 



,, 10. — Illustrates orthostereoscopism by Dr. Mercer's 

 method. The dotted lines denote the eye-spots 

 or Ramsden's circles. The figure shows that 

 when the tubes are racked in the eye-spots are 

 brought closer together, so that the outer portions 

 of the eye-spots pass through the inner portions 

 of the pupils. In opposition to this Abbe and 

 the editorial staff at the R. M. S. state that 

 orthostereoscopism is due to the employment of 

 the outer halves of the pupils. The cause of 

 orthostereoscopism in Dr. Mercer's experiment 

 as well as in the Abbe eye-piece is due to the 

 fact that the suppression of portions of the eye- 

 spots effects a " cross-over." 



