There are relatively few antique photographs of the Sun compared to drawings of the solar disc. We want to highlight three solar photographs taken by Giorgio Roster in 1893. Roster (1843-1894) was a doctor, chemist, and photographer. The library of the Museo di storia della scienza of Florence preserves part of Roster's legacy in the section "Carte e raccolta fotografica Roster", in which we want to highlight the title "Telefotografie Roster V: Macchie solari; teleobiettivo Roster, 1892-1893". This title contains three solar photographs taken from Florence with a Roster tele-objective. The main characteristics of the three photographs are: (i) Date: 21st April 1893. Time: 5:15 p.m. Magnification: 61x. (ii) Date: 25th April 1893. Time: 4:40 p.m. Magnification: 68x. (iii) Date: 30th April 1893. Time: 17:00 p.m. Magnification: 71x. It may be interesting to note that, although there are drawings of the solar disc in the collection of the Kalocsa Observatory for the days 21 and 30, there is no such thing for day 25. Thus, Roster's photographs fill that gap in the graphical information of the Sun.
To the Editors of 'The Observatory'
A great effort has been made in the last twenty years to obtain a better reconstruction of the Sunspot Number using historical observations that cover the last four centuries [1][2][3][4] .
Moreover, spatial information about sunspots in the past is being recovered by several teams. The information includes Butterfly diagrams [5][6] , sunspot catalogues 7 , and even sunspot group tilt records 8 .
The use of antique solar photographs to increase our databases or simply to check wellknown data 9 is of great interest. However, there are relatively few photographs of the Sun compared to drawings of the solar disc 10 . Therefore, we want to highlight three solar photographs taken by Giorgio Roster in 1893. Roster (1843-1894) was a doctor, chemist, and photographer. He was interested in the usefulness of the photography in sciences and in works on microphotography and telephotography 11 It is important to note that there are some defects in the objective that could be confused with sunspots, although these imperfections are easily identifiable comparing the three photographs. In any case, sunspot groups are visible just coinciding with its heliographic coordinates provided by the GPR. In addition, the photographs are not correctly oriented. The solar north will point to the top of the photographs if they are rotated -81º, 99º, and 99º respectively. Finally, it may be interesting to note that, although there are drawings of the solar disc in the collection of the Kalocsa Observatory 12 for the days 21 and 30, there is no such thing for day 25. Thus, Roster’s photographs fill that gap in the graphical information of the Sun.
We appreciate the support of EU, Junta de Extremadura, and Ministry of Economy and Competitiveness (consortium IMDROFLOOD, Research Group Grant GR15137, IB16127, and AYA2014-57556-P).
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