COMMON THINGS. 



SPECTACLES. 



1. Their general utility. 2. Should therefore be generally understood. 

 3. Vision. 4. Blindness. 5. Defective sight. 6. Long and short 

 sight. 7. Remedy. 8. Spectacles. 9. Effect of convex lenses. 

 10. Of concave lenses. 11. Focal length of a lens. 12. Varies with 

 distance of object. 13. Peculiarities of vision in short sight explained. 

 14. In long sight. 15. The mounting of spectacles. 16. Periscopic 

 spectacles. 17. Both eyes have not always the same power of vision. 

 18. Ophthalmometer. 19. Application of it in selecting glasses. 

 20. Curious defects of vision. 



1. SPECTACLES are incontestably the most universally useful gift 

 which, optical science has conferred on mankind. More wonderful 

 instruments abound. The miracles disclosed to human vision by 

 the telescope and the microscope are known to all. To such 

 marvels, spectacles lay no claim. But to compensate for this 

 LARDNER'S MUSEUM OF SCIENCE. o 193 



No. 65. 



