Elements of natural philosophy (Volume 2-3) . d by lenses, intersecting the image in j? and q, and the axis inX; then will p q be the extent of the image of whichall the parts will appear equally bright. Draw qEQandp EP\ the angle PEQ=pEq^is the field of view,which will be denoted by I ; Field of view construction; First form of itsvalue; 23f (76) ELEMENTS OF OPTICS. 249 but P2 on n .Xr (77) Transformations; to find X 0 and Xr, call the diameter 31JV of the objectlens a, that of the eye lens /3, and we have a:/3 \\EX\XO a + /3:/3 ::JSX+XO:XO Proportions; hence, XO=P.(f+(FJ); a+ (J and in the s
Elements of natural philosophy (Volume 2-3) . d by lenses, intersecting the image in j? and q, and the axis inX; then will p q be the extent of the image of whichall the parts will appear equally bright. Draw qEQandp EP\ the angle PEQ=pEq^is the field of view,which will be denoted by I ; Field of view construction; First form of itsvalue; 23f (76) ELEMENTS OF OPTICS. 249 but P2 on n .Xr (77) Transformations; to find X 0 and Xr, call the diameter 31JV of the objectlens a, that of the eye lens /3, and we have a:/3 \\EX\XO a + /3:/3 ::JSX+XO:XO Proportions; hence, XO=P.(f+(FJ); a+ (J and in the same manner, Relations fromthe figure; Xr=f-EX=f- J^.(f+{Flt)) = Wa *<f\ these values in Equation (77), give and this in Equation (76), gives, by introducing thepowers of the lenses, Substitutions; = LX /3 I — a L l + Z • • • . (78) Final value forfield of view. The rays of each of the several pencils emerging fromthe eye lens parallel, will be in condition to afford dis- 250 NATURAL PHILOSOPHY. Fie:. 51. Geometricalillustration;. Proper positionfor the eyeindicated intelescopes ; Distance ofoptical centre ofthe eye from thatof the eye lens ; Position of theeye for theGalileantelescope ; Arrangement forchanging thedistance betweenthe lenses. tinct vision, and tlie extreme rays m 0 and n 0\ willbe received by an eye whose optical centre is situatedat 0. If the eye be at a greater or less distance than0\ from the eye lens, these rays will be excluded, andthe field of view will be contracted by an improper posi-tion of the eye. It is on this account that the tube con-taining the eye lens of a telescope usually projects ashort distance -behind to indicate the proper position forthe eye. From the similar triangles p 0 q and m Of n, we have Oa=^.rO=fi(Z + ll.{F„) .... (79) <pq pi — a L This also applies to the Galilean instrument, by chang-ing the sign of Z, which will render 0 0\ eye should, therefore, be in front of the eye-glassin order that it may no
Size: 2629px × 951px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, booksubjectmechanics, booksubjectopticsandphoto
