SCIENCE-GOSSIP. 



93 



( IIAI'TKKS FOR VOUNG ASTRONOMERS. 



Bv Frank C. DhSNKrr. 



TEI-ESCOPIC APH.\RATl"S. 



iCi'iitimu-il /miti /, Oij, 



TiiKEE kinds of eyepieces are in common use wilh 

 tile astro-telescope— the Negative or Iliiyghenian, the 

 I'ositive or Ramsilen, and the Reiner. Tliey each 

 consist of two planoconvex lenses, that next the eye 

 lieing known as the eye-lens, and the other as the field 

 lens. The first is the best for general use. The two 

 lenses are placed wilh their plane sides toward the 

 eye. Their foci are as I : J, anil they are placed half 

 the sum of their focal length apart, in other words 2. 

 The diaphragm is placed niitlwa}" lielwcen the lenses 

 in the focus of the eye-lens. To find the focal length 

 of a negative eyepiece, divide twice (he product of 

 ihe focal length of the component lenses hy their 

 sum. Thus if eye-lens equal i-3rd inch, and field- 

 lens eijual I inch, the matter stands thus : — 



2X-3.-iXi 



•5 inch. 



The Kanisden eyepiece is generally used for finders 

 and micrometers, as it has a very flat field of view. 

 Two lenses having their convex sides facing each 

 other are ])laced at a distance apart less than the focal 

 length of the eye-lens, and the diaphragm is at the 

 focus of the combination. If there be any difl'eronce 

 in the focal length of the lenses, the shorter should be 

 the eye - lens. Divide the product of 

 the focal length of the component lenses 

 by their sum, less the distance between 

 ihem, the result being their equivalent 

 focus. Thus A-inch and i'-inch lenses 

 [ inch apart will be calculated thus : — 





■3-5 i"ch. 



exactly in focus. A narrow bar put across the 

 diaphragm will often be useful to hide a bright star 

 or planet when looking for a faint companion or 

 satellite. 



Kor measuring, there are four different kinds of 

 micrometer. The ring micrometer is a ring sus- 

 pende<l in the field of view. It is necessary for the 

 observer to know its precise diameter and thickness. 

 Then by exactly noting the difference in time of two 

 objects passing behind, it is possible to calculate the 

 difference of position between them. This form ran 

 be employed with any telescope. 



The filar micrometer has two frames, having line 

 wires or spider lines, which can by means of a screw 

 be made to coincide, or to separate to a considerable 

 distance. The value of one revolution of the screw 

 has to be detei mined, then by setting the two wires 

 to intersect two objects whose distance is desired to 

 be measured, the number of revolutions, and part ol 

 a revolulion, the screw has made, is read oft' and the 

 actual distance is easily calculated. An equatorial 

 stand, with clock work, is a necessity for the filar 

 micrometer. It is also necessary at night time to 

 illuminate the field of view. 



The double image micrometer is made sometimes 

 like a day eyepiece, the third lens of which, from the 

 eye end, is divided into two eijual portions, one of 

 which slides past, the other by means of a screw. 



iiwiiiiniiiiimiiiiiiiiiiiwii !t«i\ nil iiwl imiii 



The Kelner eyepiece is only used where 

 a very large field of view is required, as for 

 nebulae, clusters, and comets. The eye- 

 lens should be achromatic, and the field-lens 

 is so placed that its cell forms the border 

 of the field of view. Both lenses have their flat sides 

 next the eye. The focus of the eye-lens is the effective 

 focus of the combination. In some of the best eye- 

 pieces the eye-lens is meniscus. 



The Harlow lens is a concave achromatic lens, 

 which if it be placed about five inches behind an eye- 

 piece increases its power Ijy about one-half. Thus if 

 a telescope has two eyepieces, 70 and 180, with a 

 Harlow lens, powers of 105 and 270 may also be 

 obtained. In practice, personally, I do not care for 

 this lens. It is, however, very useful for use with the 

 filar micrometer, increasing the power without 

 magnifying the wires. 



.■\ Star Diagonal is a right angled totally reflecting 

 prism to enable the observer to look at objects near 

 the zenith without craning one's neck. .-\ solar 

 diagonal reflects the image of the sun from the front 

 surliice of a piece of glass, which, of course, rcfiacls 

 most of the light and heat. These two are sometimes 

 combined in one, using the same prism in a reversed 

 position. 



It is sometimes useful to have cross-wires to one of 

 the eyepieces. It the diaphragm can be removed 

 this can easily be done. Stretch the spider line or 

 piece of raw silk between two pieces of cork on the 

 points of a pair of compasses, then lay them across 

 the diaphragm, fastening them with melted beeswax. 

 When set, cut off close to the metal. Be careful in 

 replacing the diaphragm to set it with the "wires" 



RED cnEEN 



VIOLET 



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l^KOWI.XlNti's, MaCL'i.E AN S ^1 



I.L I KOSCOPK 



When the two halves are coincident and the index 

 reads o, there is only one image seen ; but on turning 

 the screw two are visible, and it is easy to measure 

 in this manner planets and double stars. It is not so 

 satisfactory for lunar objects. No illumination of the 

 field is necessary and the micrometer does not so 

 much need clock work. Sometimes the double 

 image is obtained by means of a divided Barlow lens, 

 or by the use of double-refracting spar. 



The fourth form of micrometer is a scale engraved 

 on glass. Say eleven eijui-distant lines with eleven 

 others at right angles to them, whilst a diagonal 

 crosses between the tenth and eleventh. The value 

 of the divisions is accurately found, and measures of 

 distance of considerable delicacy can readily be made. 



With either a filar, double-image, or scale micro- 

 meter, a positiim circle may be used. Then not only 

 the distance separating two objects can be measured, 

 but the angle a line joining them makes 

 with the meridian, can be determined. The 

 bottom or north point of the field of view — with an 

 equatorial mounting — is marked zero. The right 

 hand, eastern or foll(noiii!; side is marked 90', the 

 top, or south, iSo , and the left, western, or /'nicJin:; 

 side 270 \ I have very successfully used the position 

 circle without any other form of micrometer to 

 triangulate portions of the moon"s surface. Equatorial 

 motion is imperative. 



( To I'C contiiittt'ri.) 



I 



