22 THE POSSIBILITIES OF THE TELESCOPE. 



of the primitive telescope, is prismatic. The effect is that the 

 red rays are brought to focus at R, and the voilet to a shorter 

 focus at V, the other tints lying between. Thus there is no 

 definite focus (See Fig. 3). 



We have, then, two effects from the passage of a ray 

 through a prism — the bending or refraction, and the decompos- 

 ing or dispersion. But the grand discovery was made that 

 different kinds of glass disperse or spread the colours in 

 different degrees with a given amount of refraction, thus : — 

 If the former prism had been of flint glass instead of plate, 

 and made to give the same amount of bending, we should 

 have had the different colours spread over, perhaps, double 

 the space. To produce an equal spreading of the colours we 

 shall require a. flint glass prism that will bend or refract less. 

 We shall require to bend the ray only to B instead of to A 

 (Fig. 2) ; that is, we shall require a thinner or less angular 

 prism. Now, place these two prisms together in reverse posi- 

 tions, and what will be the effect ? The ray will be partially- 

 brought back towards the straight, viz., to C instead of to A 

 (See Fig. 2 b), leaving still an amount of deviation, from the 

 greater thickness of the plate glass. But the dispersion of 

 the colours is in reverse order in consequence of the inversion 

 of the prisms, but in equal degree. The complimentary 

 colours are therefore superimposed, and are re-composed into 

 white light. This is the secret of the vast advance in telescopic 

 power since the days of Galileo. 



Now to apply this to the telescope. The lens (P) of plate 

 glass (see Figure 4) is backed by a concave of flint glass (G) 

 of less refraction power, thus leaving a surplus of refraction 

 to the convex, the effect being that we get a focus at F instead 

 of at /, with all the different coloured rays brought to a 

 common focus. 



Now, there is one trouble that has not yet been sur- 

 mounted. Although the general prismatic dispersion may be 

 equalised and neutralised, the colours are unequally dis- 

 tributed by the two kinds of glass. Consequently one 

 particular coloured ray does not meet its exact complimentary 

 in the reversed spectrum, and as a result we have a fringe of 

 outstanding colour. This is called the " irrationality " of the 

 spectrum. This is what opticians are endeavouring to get rid 

 of by the invention of a glass that will locate the different 

 colours in the same relative position as does the plate glass of 

 the convex, and this is all the magic there is about it. 



Possible Power of the Telescope. 



I think many persons entertain very exaggerated ideas of 



what the telescope in its present stage can really accomplish. 



Theoretically it is assumed that the utmost available power 



of the most perfect instrument/and under the most favourable 



