The telescope and microscope / by Thomas Dick.

  • Thomas Dick
Date:
[1851?]
Catalogue details

Licence: Public Domain Mark

Credit: The telescope and microscope / by Thomas Dick. Source: Wellcome Collection.

    39/204 page 31
    to A d, the object lens. The rays passing through this lens will form an image of the object in its focus at i m, and as the rays cross each other it will be inverted, e y, the eye glass, is placed exactly at its focal distance from this image, and must be of a much shorter focal distance than the object glass. The magnifying power in this telescope is in the proportion of the focal distance of the object glass to that of the eye glass. Suppose the object glass to be of 24 inches focal distance, and the eye glass 1 inch, the magnifying power is then as 1 to 24 ; in other words, it makes a distant object appear 24 times nearer, and 24 times larger in size than it is to the naked eye. If the eye glass had been li inch focal distance, the magnifying power would have been only 16 times. Through this telescope all objects appear inverted, like the image at i m. This telescope was much in use by astro- nomers in the seventeenth and eighteenth cen- turies. In order to obtain a considerable mag- nifying power, it was sometimes made more than a hundred feet long ; and as it was found inconvenient to have a tube of this length, the telescope was used without one. A high pole was erected, the object glass was placed at the top of it, and capable of being turned in every
    digitised image 1
    digitised image 2
    digitised image 3