RET 



IIET 



0sly been exposed to it. On closing 1 

 the eyes after viewing- the black spot 

 on the white paper, a red spot is seen 

 of the form of the black spot ; for that 

 part of the retina on which the figure 

 of the black spot was formed, being 

 more sensible to the light than the 

 other parts, is capable of being 1 brought 

 into action by the red ra\s which pene- 

 trate the eye-lids. Upon the same prin- 

 ciple Dr. R. Durwin accounts for the fol- 

 lowing fact. A writer in the Berlin Me- 

 moirs observes, that when he held a 

 book, so that the sun shone upon his 

 half closed eye-lids, the black letters 

 which he had long inspected, became 

 red. There is a similar story told by 

 Voltaire of a Duke of Tuscany, who 

 was playing at dice with a general of a 

 foreign army, and believing that he saw 

 red spots on the dice, portended dread- 

 ful events, and retired in confusion. 

 The observer, after looking for a minute 

 on the black spots of a die, in a bright 

 day, and carelessly closing his eyes, 

 would see red spots corresponding to 

 the black spots on the die, and if they 

 were intense, from the fatigue or weak- 

 ness of the optic organ, those appearances 

 would continue, and on looking at the 

 die, would be supposed to be upon it, 

 just as before stated ; persons in a very 

 weak state often see black spots which 

 they refer to the bed clothes. 



RE7TCULA, or Rr/rieci.E, in astrono- 

 my, a contrivance for the exact measur- 

 ing the quantity of eclipses. The reticule 

 is a little frame, consisting of thirteen 

 fine silken threads, equidistant from each 

 other, and parallel, placed in the focus of 

 object-glasses of telescopes ; that is, in 

 the place where the image of the lu- 

 minary is painted in its full extent ; of 

 consequence, therefore, the diameter of 

 the sun or moon is hereby seen divide:! 

 into twelve equal parts or digits ; so that 

 to find the quantity of the eclipse, there 

 is nothing to do but to number the lumi- 

 nous and the dark parts. As a square 

 reticule is only proper for the diameter, 

 not for the circumference, of the lumina- 

 ry, it is sometimes made circular by 

 drawing six concentric equi-distant cir- 

 cles. This represents the phases of the 

 eclipse perfectly. 



RETINA, in anatomy, the expansion of 

 the optic nerve on the internal surface of 

 the eye, whereupon the images of ob- 

 jects being painted, are impressed, and 

 by that means conveyed to the common 

 sensory in the brain, where the mind 



views and contemplates their ideas. See 

 OPTICS. 



KCTORNO habendo, in law. See RE- 

 PLEVIN-. 



RETORT. See LABOHATORY. 



RETRAX1T, in. law, is where the 

 plaintiff or demandant comes in person 

 into the^court, and says he will proceed 

 no further ; and this is a b >.r of all other 

 actions of like or inferior nature. 



RETRENCHMENT, in the art of war, 

 any kind of work raised to cover a post, 

 and fortify it against the enemy, such as 

 fascines loaded with earth, gabions, bar- 

 rels of earth, sand-bags, and generally all 

 things that can cover the men and stop 

 the enemy. But retrenchment is more 

 particularly applicable to a foss bordered 

 with a pai-apet; and a post fortified thus 

 is called post retrenched, or strong post. 

 Retrenchments are either general or par- 

 ticular : general retrenchments are new 

 fortifications made in a place besieged, 

 to cover the besiegers when the enemy 

 become masters of a lodgment on the 

 fortification, that they may be in a condi- 

 tion of disputing the ground inch by 

 inch, and of putting a stop to the ene- 

 my's progress, in expectation of relief. 

 Particular retrenchments are such as are 

 made in the bastions, when the enemy 

 are masters of the breach. These can 

 never be made but in new full bastions, 

 for in empty, or hollow ones, there can 

 only be made retirades. The particular 

 retrenchments are made several ways, 

 according to the time they have to cover 

 themselves : sometimes they are made 

 before-hand, which are certainly the best. 

 The parapets of such retrenchment* 

 ought to be five or sixfeet thick, and five 

 feet high, with a large and deep foss, 

 from whence ought to run out small fou- 

 gades and countermines. 



RETROGRADATION, in astronomy, 

 is an apparent motion of the planets, by 

 which they seem to go backwards in the 

 ecliptic, and to move contrary to the or- 

 der of the signs, as from Aries to Taurus; 

 from Tauinis to Gemini, &c. which, from 

 west to east, is said to be direct. When 

 it appears for some days in the same 

 place or point in the heavens, it is said 

 to be stationary ; and when it goes in an- 

 tecendentia, or backwards, or contrary 

 to the order of the signs, which is from 

 east to west, it is said to be retrograde. Sa- 

 turn continues retrograde about 140 days; 

 Jupiter 120 ; Mars 73 ; Venus 42 ; and 

 Mercury 22. The interval between two 

 retrogradations of the several planets are 

 as follow : 



