The Modern Era of the Telescope

The last speculum of note to be constructed was one four feet in diameter, made by Grubb in 1870 for the Melbourne Observatory. Silver-on-glass mirrors replaced the more expensive and difficult-to-work speculum.

Silver was far more efficient as a reflector, for a freshly deposited coat reflected somewhat better than 90 percent of the light in the visible spectrum. It tarnished, but not nearly so quickly as speculum, and it could be removed by chemical means and a new coating applied without upsetting the figure of the glass surface.

Now, accompanied by Foucault's method of testing, reflecting telescope making by amateurs began to spread, slowly at first, throughout the civilized world. Books and articles on astronomy frequently contained instructions on the subject. Almost any thick disk of glass stood a fair chance of terminating its career at the bottom of a telescope tube. Porthole lights were especially preferred on account of their ready-made size.

The secret of the achromatic was still the guarded possession of the professional optician, but simple lenses, in sets, and achromats as well continued to be offered on the market. Prices taken from catalogues of the year 1878 were, for single lenses, from two to six dollars for diameters of 1½" t0 3". The higher-priced assortment included a "genuine Huygens eyepiece mounted in brass." Achromatic lenses, diameter 2", focal length 30", sold for $4.50,
and 4-inch achromatic lenses of 60" focal length, with "extra fine finish," were priced at 75 dollars.
 

Modern era of the telescope continued