1890.] on Astronomical Telescopes. 163 



illumination will, by its cumulative effect, make a photographic 

 picture, so that there are ways in which the photographic method of 

 seeing celestial bodies can be possibly made superior to the direct 

 method of looking with a telescope. 



With some celestial objects this has been already done : stars too 

 faint to be seen have been photographed, and nebulae that cannot be 

 seen have also been photographed; but much more than this is 

 possible : we may be able to obtain photographs of the surface of the 

 moon similar to those I have shown, but on a very much larger scale, 

 and we may obtain pictures of the planets that will far surpass the 

 pictures we would see by the telescope alone. 



I have mentioned that the distance at which the normal eye can 

 best see things is about nine inches, as that gives the greatest angular 

 size to the object while retaining a sharp picture on the retina ; but, 

 as many of us know, eyes differ in this power : two of the common 

 infirmities of the eyes are long or short-sightedness, due to the 

 pictures being formed behind the retina, in the first case, and in front 

 of it in the other. Towards the end of the thirteenth century it was 

 found that convex lenses would cure the first infirmity, and, soon 

 afterwards, that concave lenses would cure the second, as can be easily 

 seen from what I have said about the action of these lenses ; so that 

 during the fifteenth and sixteenth centuries the materials for the 

 making of a telescope existed ; in fact, in the sixteenth century. Porta 

 invented the camera obscura, which is in one sense a telescope. It 

 seems very strange that the properties of a convex and concave lens 

 when properly arranged were not known much earlier than 1608. 

 Most probably, if we may judge from the references made by some 

 earlier writers, this knowledge existed, but was not properly appre- 

 ciated by them. Undoubtedly, after the first telescopes were made in 

 Holland in 1608, the value of this unique instrument was fully 

 appreciated, and the news spread rapidly, for we find that in the next 

 year " Galileo had been appointed lecturer at Padua for life, on 

 account of a perspective like the one which was sent from Flanders 

 to Cardinal Borghese." As far as can be ascertained, Galileo heard 

 of the telescope as an instrument by which distant objects appeared 

 nearer and larger, and that he, with this knowledge only, reinvented 

 it. The Galilean telescope is practically, though not theoretically, 

 the simplest form. It is made of a convex lens in combination with a 

 concave lens to intercept the cone of rays before they come to a focus, 

 and render them parallel so that they can be utilised by the eye. It 

 presents objects as they appear, and the picture has less colour in 

 this form than in the other where a convex eye-glass is used. It is 

 used as one form of opera-glass at the present time. Made of one piece 

 of glass in the shape of a cone, the base of which is ground convex, and 

 the apex slightly truncated and ground concave, it becomes a single- 

 lens telescope that can be looked upon just as an enlargement of the 

 outer lens of the eye. 



Galileo was undoubtedly the first to make an astronomical 



M 2 



