420 



KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1906. 



quired. When it is desired to secure an enlarged photo- 

 graph of the whole of the solar or lunar disc, the 

 diameter of the lens must, of course, be at least equal 

 to that of the primary image. To bring such lenses 

 into use some form of camera is necessary. This can 

 be attached to the eye-tube of the telescope in the same 

 manner as the carrier previously referred to. Fig. 3, 

 which is self-explanatory, shows the complete arrange- 

 ment as used by the writer. The enlarging lens is 

 fixed in a sliding tube, and, through the small door with 

 which the camera is fitted, it can be moved nearer to or 

 further from the plate to increase or diminish the 

 amount of magnification, the focussing being accom- 

 plished by the rack-work of the telescope. For 

 planetary work a screw similar to that pre\iously de- 

 scribed, and which is also shown in the figure, can be 

 brought into use, and se\eral exposures put on the 

 same plate. 



Shifters. — It will be found of great convenience for 

 regulating the exposure if some form of shutter is 

 attached to the eye-tube of the telescope. The type 

 adopted should have a fairly large opening, work 

 smoothly, and admit of giving " time " and " instan- 

 taneous " exposures. It should be fitted with a short 

 length of brass tube similar to that described for the 

 camera and plate-c;irrier. It mav then be readily placed 

 in position by sliding the tube into the end of the eye- 

 tube which is next the flat, and, consequently, inside 

 the telescope, the release being most conveniently 

 effected bv the usual pneumatic ball. 



Drivi7ig-Clock\. — Manv amateurs possess driving- 

 clocks, but only a small proportion of these give suffici- 

 ently accurate results to enable their owners to take up 

 photographic work in a svstematic fashion. In some 

 cases failure may arise from faulty construction of the 

 mount. The polar axis may be too short to secure the 

 requisite rigidity, or, what is a more common defect, 

 the driving-worm, or pinion, as the case may be, is 

 geared to too small a sector or circle. If a larg-e sector 



Fig. 4. 



be used, very little strain is put upon the driving 

 mechanism, .-ind a greater measure of steadiness is at 

 once secured. 



Fig. 4 shows a sector of two feet radius with the 

 driving-worm geared in position. It is very important 

 that both the driving-worm and sector be" accurately 



cut, otherwise slight movements of the image in the 

 field will be apparent, although the motion of the clock 

 may be perfectly regular. 



In the matter of clocks, perhaps the most frequent 

 source of trouble arises from the method of control, 

 the ordiuarv fiiilional governor being uncertain in 



Fig. s- 



action, and, where accurate driving is essential, necessi- 

 tating the introduction of electrical or other complica- 

 tions. For time-keeping purposes no method of control 

 equals the pendulum for steadiness and reliability, but 

 pendulum clocks in the ordinary form are quite useless 

 for driving astronomical instruments, for the simple 

 reason that they are fitted with escapements, which only 

 admit of motion in the form of short though regular 

 impulses, while for astronomical purposes the motion 

 must be continuous. A clock can, however, be con- 

 structed whereby this result is obtained, and the ad- 

 vantages of the pendulum control at the same time 

 secured. When the principle on which its utility de- 

 pends is once understood the constructional details mav 

 be worked out in a variety of ways. 



This principle may be made clear by referring to 

 '■'§^- S> which is intended to illustrate the action of a 

 train of wheels on a single pendulum. A represents 

 the clock barrel, B the driving weight, and C the pendu- 

 lum. When the barrel rotates, motion is conveyed 

 through the intervening wheel D to the bevel pinion E, 

 and thence to the vertical spindle F, on the upper end 

 rf which there is fitted a short crank, which, in turn, 

 communicatees its motion through the connecting-rod G 

 to the pendulum C, and as the latter swings backwards 

 and forwards it regulates the rate of motion of the 

 whole arrangement. A single pendulum will not give 

 so satisfactory results as 3, and this is the minimum 

 number recommended for a clock of the kind. 



Fig. 6 is a photograph of a 3-pendulum clock of this 

 description, which the writer found to give excellent 

 results. On the top of the vertical spindle to which the 

 crank is connected a train of .small wheels terminating 

 in a "fly" should be fitted, for the purpose of intro- 

 ducing a certain amount of flexibility in the drive be- 

 tvi-een the clock and pendulums, in order that the oscilla- 

 tory motion of the latter may be started and stopped 



