1882.] 



MICROSCOPICAL JOURNAL. 



191 



the illusion of greater distance and 

 size, for the object retinally pictured. 

 This is in accordance with the laws 

 of association ; for under ordinary cir- 

 cumstances, near vision requires con- 

 traction, and distant vision relaxation, 

 of internal rectus and ciliary muscles ; 

 while unusual contraction of the ex- 

 ternal rectus muscles is not unfre- 

 quently necessary in the ordinary use 

 of the stereoscope, involving discom- 

 fort and an illusion of increased 

 distance in the binocular picture. 



All our judgments, whether visual 

 or otherwise, become vitiated when 

 conditions are very different from 

 those to which we are most accustom- 

 ed. Prof. Brewer's 440 observers 

 accommodated their eyes, as nearly 

 as possible, to the same external con- 

 ditions. The striking diversity in the 

 conclusions reached by them, shows 

 how various were the muscular con- 

 ditions, under which they interpreted 

 their own sensations. To this must 

 be added the important fact to which 

 attention was called by him, that for 

 the same * eye much depends upon 

 education. The mechanic who 

 thought the picture looked to be five 

 feet long, and projected upon a 

 screen, was quite unaccustomed to 

 forming judgments with no actual 

 objects for comparison ; and in any 

 event there was doubtless room for 

 improvement in his visual education. 



Another striking example of varia- 

 tion in judgment by the same person, 

 under changed ophthalmic conditions 

 is found in early experiences with the 

 binocular microscope, by the original 

 inventor of this instrument, Prof. J. 

 L. Riddell, of New Orleans, La. In 

 looking with both eyes at an object 

 ten inches distant, the two visual 

 lines form an angle of a little over 

 14°, and a corresponding degree of 

 contraction of the internal rectus 

 muscles is necessitated. The two 

 tubes of Dr. Riddell's first binocular 

 microscope were sensibly parallel, 

 the sheaf of rays after passing through 

 the objective being divided, and 

 each half subjected to two reflections 



before reaching the observer's eye. 

 In a subsequent improvement a pair 

 of prisms were placed with the lower 

 edges in contact, and rays transmitted 

 with two refractions and one reflec- 

 tion, reaching the eyes in such man- 

 ner that the optic angle was less 

 than 14°. In either case, therefore, 

 to adapt the eyes to this condition 

 the internal rectus muscles were re- 

 laxed, and a slight change of adjust- 

 ment in the instrument was necessary. 

 Dr. Riddell describes the result as 

 follows : "Thus, a mite of a wheel 

 animalcule, one-hundredth of an 

 inch long, will perhaps appear to be 

 a foot off, and as large as a mouse ; 

 but bring the prisms nearer together, 

 and tilt the oculars to correspond, 

 and the image waxes marvelously 

 immense ; and taking a position per- 

 haps apparently more than a hundred 

 feet distant, the being, too small to 

 be seen with the naked eyes, vies 

 with the great whale of the ocean 

 in size : wearing an aspect more 

 awful to behold than the savage 

 beasts of the African forests ; exhibit- 

 ing a complex transparent structure, 

 more unique and wonderful than the 

 mind of man can well conceive." 



We can good-naturedly forgive a 

 little exuberance of imagination when 

 the reality which it accompanies is 

 the first revelation from such an in- 

 strument as that introduced to 

 science by Dr. Riddell. 



Death and Continuous Life in 

 the Animal World.* 



BY N. CHOLODKOWSKY. 



In No. 103 of Zoologischer Afi- 

 zeiger, for the year 1882, appeared an 

 essay by Prof. O. Biitschli (" Thoughts 

 upon Life and Death "), in which, 

 referring to the normally not occur- 

 ring death of the Protozoa (in the 

 sense we use this expression for 

 the higher animals), he started the 



* " Tod iind Unsterblichkeit in der Thier- 

 welt." Translated from Zoologischer An- 

 zeiger. 



