Sweden's King Carl XVI Gustaf (L) greets Japan's Emperor Akihito (R) and Empress Michiko (2nd R) upon their arrival at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018.

Sweden's King Carl XVI Gustaf (R), Queen Silvia (2nd R), Japan's Emperor Akihito (L) and Empress Michiko visit Special Exhibition 'The Art of Natural Science in Sweden' at the JP Tower Museum Intermediatheque in Tokyo, Japan, April 23, 2018.

Sweden's King Carl XVI Gustaf (L), Queen Silvia (2nd R) greet Japan's Emperor Akihito (2nd L) and Empress Michiko (R) upon their arrival at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018.

Sweden's King Carl XVI Gustaf (L) greets Japan's Emperor Akihito (R) at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018.

Sweden's King Carl XVI Gustaf (L), Queen Silvia (2nd R) greet Japan's Emperor Akihito (2nd L) and Empress Michiko (R) upon their arrival at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018. Kazuhiro Nogi/Pool via Reuters

Sweden's Queen Silvia (2nd L) greets Japan's Empress Michiko (2nd R) while King Carl XVI Gustaf (L) and Emperor Akihito (R) look on upon their arrival at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018.

Sweden's King Carl XVI Gustaf (L), Queen Silvia (2nd L) greet Japan's Emperor Akihito (2nd R) and Empress Michiko (R) upon their arrival at the special exhibition "The Art of Natural Science in Sweden" at the JP Tower Museum Intermediatheque in Tokyo, Japan April 23, 2018.