OPTICS. 



CHAPTER I. 



THE MISSIONS OF HISTORY.— THE CAMERA OBSCURA.— LONG 

 AND SHORT SIGHT.— STEREOSCOPE AND PSEUDOSCOPE.— 

 MULTIPLYING-GLASSES. 



The Camera Obscura. — Telescopes, Microscopes, and Spectroscopes, and their 

 separate Objects. — Structure of the Camera Obscura. — The Double Convex 

 Lens. — Its "Use as a Burning-glass. — The Meridian Gun in Paris. — Significa- 

 tion of the Word " Focus." — The Human Eye and its Analogies to the Camera 

 Obscura. — Forms of various Lenses. — Long and Short Sight. — Their Causes 

 and Means of Remedy. — Alteration of Sight in the Diver. — Long and Short 

 sighted Spectacles. — The Eye of Birds. — Its beautiful Structure. — Washing- 

 glasses and the "Nictitating" Membrane. — Combination of Images. — 

 Natural Stereoscopes. — The Pseudoscope and its Effects on an Object. — The 

 Multiplying-glass. — The Eight Eyes of the Spider and their Arrangement. — 

 The Seventy Thousand Eyes of the Butterfly.— Form of the Facets. 



HISTORY seems to fall into natural divisions, and to write the 

 records of time in successive epochs, recording the advance 

 of the human race. Some of them have apparently disappeared 

 except by the strange relics which they have left behind, but 

 though nothing is known of the men who worked in these 

 ancient times, they stamped their mark upon the earth, and 

 evidently left the world better than they found it. 



A very admirable treatise on this subject has been written by 

 the late Rev. J. Smith, called the " Divine Drama of Creation." 

 In this work he divides the progress of the human race into 

 five acts, like those of a drama. The first act is the Hebrew 

 Mission, the second the Greek Mission, the third the Roman 

 Mission and the Middle Ages, the fourth the National Mission, 

 and the fifth the Universal Mission. 



Certainly a scene of the last act is now in progress, and may 

 be entitled the Scientific Mission. The last hundred years 

 nave been indeed the age of discovery, and, during that time, 



