TELESCOPE. 



convex glafs, is equally trus of the imagf of an objeft fo 

 viewed. It is eafy, therefore, to conceive, how two Icnfes 

 of different focal lengths may be fo arranged as to make a 

 telefcope that will at the fame time invert and amplify, as to 

 fenfe, a diftant objetl : for, firll, a lens of a long focal dif- 

 taace will form a large image of the objeft oppofed to it, 

 which image, by the eroding of the rays at the focal point, 

 will be inverted a httle beyond the folar focus : and fccondly, 



viewed through the outcrmoll or firlt cyc-glafs, in the fame 

 manner as the firft or inverted image was viewed tlirongh 

 the original cye-glafs. This telefcope was denominated the 

 terrejfrial telefcope ; and while the foci of all the three eye- 

 glades were fimilar, its power and field of view remained 

 the fame as in the aftronomical telefcope. 



The theory of thefe three conftruftions will be more 

 clearly underflood by a reference to Plate XXV. of yijlro- 



an eye applied to a lens of (liort focal diilance, which is nomical Injlrumetils, in which fg. i. (hews the arrange 

 held fo that its focal point may coincide with that of the ment of the glafles in the aftronomical telefcope ; Jig. 2 

 larger lens, will receive parallel rays, and will fhew the faid '■'""' -'" -' v ,y- • , ^ ,.. , ., . . 



image in an amplified or magnified ftate, and in the fame 

 inverted pofition in which it is exhibited ; which image, by 

 being enclofed in a darkening tube, appears with all its natu- 

 ral colours. The power of fuch a telefcope, which is the 

 fimpleft that can be made, is afcertained by finding how often 

 the focal length of the fmall or eye-lens is contained in 

 tlie focal length of the larger or objeft-lens ; the quotient of 

 fuch divifion will reprefent the power. But if the eye-lens 

 be made concave, and placed within the focal point of the 

 objeft-lens, as much as is equal to the virtual focus of the 

 concave lens, then the converging rays will become parallel, 

 and afterwards, on entering the eye, which may be ccnfi- 

 dered as a lens of fliort focus, wiU converge, and form a di- 

 re6l image on the retina ; and though the total length of the 

 telefcope will be (hortened by this latter arrangement, by 

 twice the focal length of the eye -lens ; yet if the virtual 

 focus of the concave eye -glafs be the fame as the focal dif- 

 tance of the convex lens, the power will be the fame, and 

 may be afcertained by the fame procefs. With a convex 

 eye~glafs, the inftrument ariCng out of the firll arrangement 

 is the original ajlronoviical telelcope, and that arifing out of 

 the fecond is the Galilean. The field of view in the former 

 conftruiSion is direftly as the efFeftive breadth of the eye- 

 glafs, and inverfely as the interval between the lenfes ; but in 

 the latter, the field is direftly as the diameter of the pupil of 

 the eye, and inverfely as its diftance from the lens. 



In both thefe conftruftions, the fmalleft power, or, which 

 is the fame thing, the fhorteft focus of the objeft-glafs is 

 ■with parallel rays ; and as the diftance of the objeft, or ra- 

 diant point, decreafes according to the principles of optics 

 laid down under Lens, the focal diftance of the objeft-glafs 

 increafes : and thus the power increafes as the rays become 

 more and more diverging, from a gradual decreafe of dif- 

 tance ; fo that, in faft, the fame telefcope magnifies a near 

 objeft confiderably more than it does a diftant one ; for 

 wliile the focus of the objeft-glafs increafes after a certain 

 law, inverfely as the diftance decreafes, the focus of the 

 eye-glafs remains unaltered ; and, confequently, the power 

 varies inverfely as the diftance, or direftly as the variable 

 focus of the objeft-glafs. 



To remedy the inconvenience of inverfion of the objedl in 

 the aftronomical telefcope, and alfo of the contrafted field of 

 view of the Galilean, two more glafles were added to the 

 eye-tube, as we before ftated, to render the image of the 

 fjbjeft ereR, or rather to form a fecond image in a contrary 

 pofition. The primary intention of thefe two additional 

 eye-glan"es was not to alter the power, but merely to give 

 an ereft pofition to the apparent objeft ; the original lens 



that of the glafles in the Galilean, and alfo in the com- 

 mon opera-glafs, except that in it the objea-glafs i< 

 ufually achromatic ; and jig. 3. exhibits the fyftem of 

 glafles that compofe the original terreftrial telefcope, or 

 pcrfpeftive frlafs, before the fubfequent improvements took 

 place. In all thefe figures the fame letters denote the fame 

 parts, as tar as they extend ; and the magnifying power of 

 each may thus be demonftrated to be as we ha\-e before 

 ftated it. Let A E reprefent the objeft-glafs, and C D the 

 cye-glafs oi fig. i ; and let H F I and G F M be confidercd 

 as two pencils of light, proceeding in ftraight lines from the 

 oppofite ends of a diftant arrow, and crofnng each other at 

 the centre F of the faid objcft-glafs ; alfo let the dotted 

 line be a pencil coming from the middle of the arrow, and 

 falling perpendicularly on the fame central point, fo as to 

 pafs along the axis of the glafles F L E. Lender thefe cir- 

 cumftances, the angle G F H = I E M, the oppofite angle, 

 is that under which the arrow appears to the natural eye at 

 F ; but the angle I E M = C K D, is that under which 

 the image I M of the diftant arrow is viewed, when magni- 

 fied by the eye-glafs C D. But the angle I E M is to the 

 angle I F M, as L F to L E, or as the focal diftance of the 

 objeft-glafs to the focal diftance of the eye-glafs ; therefore 



LF 



Y-^ = the power, as before ftated ; and as the lenfes C D, 



N O, and T U, in^^f . 3. have equal foci, the fecondaij direft 

 image P Q is equal to the primary inverted one I M, and 

 appears under the fame angle. 



Novv if all the rays of hght had been, as they were fup- 

 pofed to be before fir Ifaac Newton's experiments, homo- 

 geneal ; and if a double convex lens, of equal curvature on 

 both fides, had been found to refraft all thefe homogeneal 

 rays into one focal point, without any aberration, either 

 lateral or longitudinal ; then the telefcopes, we have juft 

 noticed, would have been fufiiciently perfeft for all the pur- 

 pofes of exhibiting a well-defined pifture of the objeft 

 viewed in a magnified ftate ; and the power might have 

 been increafed to almoft any extent, by varying the ratio 

 between F L and E L ; that is, by incrcafing the focal 

 diftance of the objeft-glafs, or by leflening the focal diftance 

 of the eye-glafs, or by both ; but it was foon found that 

 the rays which enter a lens at or near the edges, arc re- 

 frafted to a point nearer to its furface than the rays that are 

 tranfmitted near the centre ; and alfo that the rays of dif- 

 ferent colours are differently refrafted, even from the fame 

 point of the lens, fo as to meet in the line of the axis at dif- 

 ferent diftances from the nearcft furface of the lens. The 

 former of thefe deviations, being occafioned by the fphcrical 



therefore remained as before, and was called the/cW-glafs, as figure of the lens, is called \.\\e fpherical aberration ; and the 

 being neareft to the field of view of the old arrangement of latter, arifingoutof the nature of folar light itfelf, is called the 

 two glaffes, while the next glafs was called the fecond eye- prifmatic,chrotnatic, or Newtonian aberration. The indiftinft- 



glafs, and was placed at double its focal diftance from the 

 field-glafs, fo that the rays might be parallel, and that it 

 might form another image in its focus : this being the image 

 of an image, was denominated the fecondary image, and 

 became erefl by a fecond croffing of the rays, and was then 



nefs in the formation of the image, occafioned by thefe aberra- 

 tions of the rays of light, became an objeft of fir Ifaac New- 

 ton's attention, and he foon difcovcred that, whatever mechani- 

 cal means might effc61 in the ihape of the curve that might 

 reftify the fpherical aberration, the prifmatic aberration would 

 Hh 2 remaic. 



